`tidb_enable_index_merge_join=ON` causes nondeterministic wrong results (random subset of rows) in IN-subquery queries executed by IndexMergeJoin

## Bug Report

**TiDB version:**
`8.0.11-TiDB-v8.5.8` (official docker image `pingcap/tidb:v8.5.8`)

**Steps to reproduce:**

```sql
DROP TABLE IF EXISTS ta, tb;
CREATE TABLE ta (id INT PRIMARY KEY, c0 INT, c1 INT, c2 VARCHAR(50));
CREATE TABLE tb (id INT PRIMARY KEY, c0 INT, c1 INT, c2 VARCHAR(50));
CREATE INDEX ix_tb_c0 ON tb(c0);
INSERT INTO ta VALUES (1,10,100,'a'),(2,20,200,'b'),(3,30,300,'c'),(4,NULL,400,'d'),(5,10,500,'e'),(6,20,NULL,'f'),(7,40,700,'g'),(8,50,800,'h');;
INSERT INTO tb VALUES (1,10,1000,'r1'),(2,20,2000,'r2'),(3,20,2000,'r3'),(4,10,1000,'r4'),(5,60,1000,'r5'),(6,10,1000,'r6'),(7,20,2000,'r7'),(8,10,2000,'r8'),(9,60,2000,'r9'),(10,NULL,NULL,'r10'),(11,10,2000,'r11'),(12,60,NULL,'r12'),(13,20,2000,'r13'),(14,20,NULL,'r14'),(15,10,1000,'r15'),(16,60,1000,'r16'),(17,30,NULL,'r17'),(18,20,1000,'r18'),(19,NULL,1000,'r19'),(20,60,1000,'r20'),(21,20,NULL,'r21'),(22,20,1000,'r22'),(23,10,2000,'r23'),(24,20,1000,'r24'),(25,20,1000,'r25'),(26,60,NULL,'r26'),(27,NULL,NULL,'r27'),(28,20,NULL,'r28'),(29,30,2000,'r29'),(30,20,1000,'r30'),(31,20,2000,'r31'),(32,20,416,'r32'),(33,60,NULL,'r33'),(34,20,NULL,'r34'),(35,10,2000,'r35'),(36,10,NULL,'r36'),(37,60,NULL,'r37'),(38,10,2000,'r38'),(39,30,2000,'r39'),(40,NULL,520,'r40'),(41,NULL,2000,'r0'),(42,20,2000,'r1'),(43,20,NULL,'r2'),(44,60,572,'r3'),(45,30,2000,'r4'),(46,20,598,'r5'),(47,10,1000,'r6'),(48,10,2000,'r7'),(49,20,637,'r8'),(50,10,650,'r9'),(51,60,663,'r10'),(52,20,1000,'r11'),(53,10,NULL,'r12'),(54,30,1000,'r13'),(55,20,715,'r14'),(56,20,728,'r15'),(57,10,NULL,'r16'),(58,20,1000,'r17'),(59,20,2000,'r18'),(60,20,NULL,'r19'),(61,20,1000,'r20'),(62,30,806,'r21'),(63,10,1000,'r22'),(64,30,NULL,'r23'),(65,20,1000,'r24'),(66,20,1000,'r25'),(67,10,871,'r26'),(68,10,2000,'r27'),(69,20,897,'r28'),(70,20,NULL,'r29'),(71,60,2000,'r30'),(72,20,NULL,'r31'),(73,60,NULL,'r32'),(74,NULL,962,'r33'),(75,10,2000,'r34'),(76,20,1000,'r35'),(77,30,1000,'r36'),(78,20,2000,'r37'),(79,10,1000,'r38'),(80,10,2000,'r39'),(81,10,1000,'r40'),(82,30,1000,'r0'),(83,20,NULL,'r1'),(84,NULL,2000,'r2'),(85,20,1105,'r3'),(86,20,1118,'r4'),(87,60,2000,'r5'),(88,10,1000,'r6'),(89,60,NULL,'r7'),(90,60,1170,'r8'),(91,NULL,1000,'r9'),(92,10,1000,'r10'),(93,60,NULL,'r11'),(94,10,2000,'r12'),(95,20,2000,'r13'),(96,NULL,2000,'r14'),(97,60,2000,'r15'),(98,20,1274,'r16'),(99,20,1000,'r17'),(100,NULL,1000,'r18'),(101,10,1000,'r19'),(102,10,2000,'r20'),(103,20,1339,'r21'),(104,NULL,1352,'r22'),(105,20,1365,'r23'),(106,10,2000,'r24'),(107,60,1000,'r25'),(108,60,NULL,'r26'),(109,NULL,NULL,'r27'),(110,60,1430,'r28'),(111,20,2000,'r29'),(112,20,2000,'r30'),(113,10,1000,'r31'),(114,30,1000,'r32'),(115,10,1495,'r33'),(116,20,1000,'r34'),(117,10,2000,'r35'),(118,10,1000,'r36'),(119,20,1547,'r37'),(120,10,2000,'r38'),(121,10,1000,'r39'),(122,60,NULL,'r40'),(123,20,2000,'r0'),(124,30,2000,'r1'),(125,20,1625,'r2'),(126,20,NULL,'r3'),(127,NULL,NULL,'r4'),(128,10,1000,'r5'),(129,NULL,1000,'r6'),(130,10,2000,'r7'),(131,20,2000,'r8'),(132,30,1000,'r9'),(133,20,NULL,'r10'),(134,20,2000,'r11'),(135,NULL,NULL,'r12'),(136,10,NULL,'r13'),(137,10,2000,'r14'),(138,20,1000,'r15'),(139,10,2000,'r16'),(140,20,NULL,'r17'),(141,20,NULL,'r18'),(142,20,NULL,'r19'),(143,NULL,NULL,'r20'),(144,10,1000,'r21'),(145,60,NULL,'r22'),(146,10,1000,'r23'),(147,30,2000,'r24'),(148,20,2000,'r25'),(149,NULL,1937,'r26'),(150,10,1000,'r27'),(151,10,2000,'r28'),(152,10,NULL,'r29'),(153,20,1989,'r30'),(154,20,1000,'r31'),(155,20,NULL,'r32'),(156,10,NULL,'r33'),(157,20,2000,'r34'),(158,10,NULL,'r35'),(159,60,2000,'r36'),(160,20,2000,'r37'),(161,20,2093,'r38'),(162,10,2000,'r39'),(163,30,2119,'r40'),(164,20,NULL,'r0'),(165,20,1000,'r1'),(166,60,1000,'r2'),(167,NULL,2000,'r3'),(168,20,2184,'r4'),(169,30,NULL,'r5'),(170,20,2000,'r6'),(171,10,2000,'r7'),(172,60,NULL,'r8'),(173,20,1000,'r9'),(174,20,NULL,'r10'),(175,60,NULL,'r11'),(176,10,1000,'r12'),(177,20,2000,'r13'),(178,20,1000,'r14'),(179,10,2327,'r15'),(180,30,NULL,'r16'),(181,60,1000,'r17'),(182,60,2000,'r18'),(183,20,1000,'r19'),(184,60,1000,'r20'),(185,20,NULL,'r21'),(186,60,1000,'r22'),(187,30,NULL,'r23'),(188,10,NULL,'r24'),(189,20,2457,'r25'),(190,30,2470,'r26'),(191,20,2000,'r27'),(192,20,2496,'r28'),(193,60,2000,'r29'),(194,20,2522,'r30'),(195,10,NULL,'r31'),(196,20,2000,'r32'),(197,60,NULL,'r33'),(198,NULL,2574,'r34'),(199,NULL,2000,'r35'),(200,NULL,2000,'r36'),(201,10,NULL,'r37'),(202,30,1000,'r38'),(203,20,NULL,'r39'),(204,20,2000,'r40'),(205,20,1000,'r0'),(206,10,2000,'r1'),(207,NULL,1000,'r2'),(208,NULL,2704,'r3'),(209,20,2717,'r4'),(210,NULL,2730,'r5'),(211,20,2000,'r6'),(212,10,1000,'r7'),(213,60,2000,'r8'),(214,20,2782,'r9'),(215,10,2000,'r10'),(216,10,2808,'r11'),(217,20,2821,'r12'),(218,30,2834,'r13'),(219,60,2847,'r14'),(220,20,2860,'r15'),(221,NULL,NULL,'r16'),(222,20,NULL,'r17'),(223,NULL,2000,'r18'),(224,20,2912,'r19'),(225,10,NULL,'r20'),(226,20,NULL,'r21'),(227,30,NULL,'r22'),(228,10,2000,'r23'),(229,20,2000,'r24'),(230,60,2000,'r25'),(231,20,1000,'r26'),(232,60,3016,'r27'),(233,30,3029,'r28'),(234,60,1000,'r29'),(235,20,3055,'r30'),(236,60,1000,'r31'),(237,60,3081,'r32'),(238,10,NULL,'r33'),(239,20,3107,'r34'),(240,60,2000,'r35'),(241,NULL,2000,'r36'),(242,20,3146,'r37'),(243,NULL,1000,'r38'),(244,60,NULL,'r39'),(245,60,2000,'r40'),(246,NULL,2000,'r0'),(247,10,3211,'r1'),(248,10,1000,'r2'),(249,60,2000,'r3'),(250,NULL,2000,'r4'),(251,10,NULL,'r5'),(252,60,NULL,'r6'),(253,20,3289,'r7'),(254,30,NULL,'r8'),(255,60,NULL,'r9'),(256,60,NULL,'r10'),(257,10,3341,'r11'),(258,10,1000,'r12'),(259,30,2000,'r13'),(260,10,1000,'r14'),(261,10,2000,'r15'),(262,20,1000,'r16'),(263,20,2000,'r17'),(264,20,3432,'r18'),(265,10,2000,'r19'),(266,NULL,NULL,'r20'),(267,30,2000,'r21'),(268,10,2000,'r22'),(269,20,1000,'r23'),(270,10,NULL,'r24'),(271,60,3523,'r25'),(272,20,1000,'r26'),(273,20,1000,'r27'),(274,20,1000,'r28'),(275,10,NULL,'r29'),(276,60,NULL,'r30'),(277,10,NULL,'r31'),(278,10,3614,'r32'),(279,NULL,1000,'r33'),(280,60,NULL,'r34'),(281,NULL,2000,'r35'),(282,60,2000,'r36'),(283,20,3679,'r37'),(284,NULL,3692,'r38'),(285,20,NULL,'r39'),(286,20,1000,'r40'),(287,20,3731,'r0'),(288,20,3744,'r1'),(289,60,NULL,'r2'),(290,10,3770,'r3'),(291,30,2000,'r4'),(292,NULL,2000,'r5'),(293,30,NULL,'r6'),(294,30,NULL,'r7'),(295,20,1000,'r8'),(296,20,1000,'r9'),(297,20,3861,'r10'),(298,NULL,2000,'r11'),(299,NULL,3887,'r12'),(300,NULL,1000,'r13'),(301,10,NULL,'r14'),(302,20,3926,'r15'),(303,20,NULL,'r16'),(304,30,3952,'r17'),(305,30,3965,'r18'),(306,30,NULL,'r19'),(307,NULL,NULL,'r20'),(308,20,NULL,'r21'),(309,20,1000,'r22'),(310,20,NULL,'r23'),(311,10,2000,'r24'),(312,20,2000,'r25'),(313,NULL,NULL,'r26'),(314,20,1000,'r27'),(315,20,NULL,'r28'),(316,20,NULL,'r29'),(317,20,NULL,'r30'),(318,10,2000,'r31'),(319,20,1000,'r32'),(320,10,NULL,'r33'),(321,20,4173,'r34'),(322,10,1000,'r35'),(323,20,4199,'r36'),(324,NULL,4212,'r37'),(325,30,2000,'r38'),(326,10,NULL,'r39'),(327,NULL,1000,'r40'),(328,10,4264,'r0'),(329,NULL,1000,'r1'),(330,10,2000,'r2'),(331,20,NULL,'r3'),(332,10,2000,'r4'),(333,10,4329,'r5'),(334,20,4342,'r6'),(335,NULL,4355,'r7'),(336,20,4368,'r8'),(337,20,1000,'r9'),(338,10,2000,'r10'),(339,20,NULL,'r11'),(340,10,2000,'r12'),(341,20,2000,'r13'),(342,10,2000,'r14'),(343,10,4459,'r15'),(344,NULL,4472,'r16'),(345,20,1000,'r17'),(346,20,NULL,'r18'),(347,20,4511,'r19'),(348,10,2000,'r20'),(349,20,2000,'r21'),(350,60,1000,'r22'),(351,20,2000,'r23'),(352,20,2000,'r24'),(353,10,1000,'r25'),(354,20,NULL,'r26'),(355,20,4615,'r27'),(356,10,4628,'r28'),(357,20,4641,'r29'),(358,20,4654,'r30'),(359,NULL,1000,'r31'),(360,10,4680,'r32'),(361,30,NULL,'r33'),(362,10,1000,'r34'),(363,20,1000,'r35'),(364,20,1000,'r36'),(365,20,4745,'r37'),(366,NULL,NULL,'r38'),(367,20,4771,'r39'),(368,NULL,1000,'r40'),(369,NULL,NULL,'r0'),(370,60,NULL,'r1'),(371,30,1000,'r2'),(372,20,NULL,'r3'),(373,60,4849,'r4'),(374,20,4862,'r5'),(375,10,4875,'r6'),(376,10,NULL,'r7'),(377,20,NULL,'r8'),(378,10,4914,'r9'),(379,10,4927,'r10'),(380,60,1000,'r11'),(381,20,NULL,'r12'),(382,NULL,4966,'r13'),(383,10,2000,'r14'),(384,20,2000,'r15'),(385,20,5005,'r16'),(386,NULL,1000,'r17'),(387,10,2000,'r18'),(388,20,NULL,'r19'),(389,10,5057,'r20'),(390,10,2000,'r21'),(391,10,5083,'r22'),(392,10,2000,'r23'),(393,NULL,NULL,'r24'),(394,20,5122,'r25'),(395,NULL,5135,'r26'),(396,20,5148,'r27'),(397,20,5161,'r28'),(398,20,2000,'r29'),(399,10,NULL,'r30'),(400,60,NULL,'r31'),(401,20,1000,'r32'),(402,20,NULL,'r33'),(403,NULL,2000,'r34'),(404,NULL,5252,'r35'),(405,30,NULL,'r36'),(406,60,5278,'r37'),(407,30,2000,'r38'),(408,20,5304,'r39'),(409,20,5317,'r40'),(410,10,NULL,'r0'),(411,NULL,5343,'r1'),(412,60,2000,'r2'),(413,NULL,1000,'r3'),(414,20,NULL,'r4'),(415,10,5395,'r5'),(416,10,5408,'r6'),(417,10,2000,'r7'),(418,60,2000,'r8'),(419,10,5447,'r9'),(420,20,NULL,'r10'),(421,60,1000,'r11'),(422,30,5486,'r12'),(423,30,5499,'r13'),(424,10,1000,'r14'),(425,20,NULL,'r15'),(426,20,1000,'r16'),(427,60,1000,'r17'),(428,10,5564,'r18'),(429,20,NULL,'r19'),(430,10,1000,'r20'),(431,30,1000,'r21'),(432,20,NULL,'r22'),(433,30,5629,'r23'),(434,20,2000,'r24'),(435,60,2000,'r25'),(436,20,2000,'r26'),(437,10,5681,'r27'),(438,20,5694,'r28'),(439,20,2000,'r29'),(440,NULL,NULL,'r30'),(441,NULL,NULL,'r31'),(442,10,5746,'r32'),(443,20,2000,'r33'),(444,10,5772,'r34'),(445,30,NULL,'r35'),(446,60,NULL,'r36'),(447,NULL,NULL,'r37'),(448,20,5824,'r38'),(449,NULL,1000,'r39'),(450,20,5850,'r40'),(451,30,NULL,'r0'),(452,20,1000,'r1'),(453,60,NULL,'r2'),(454,10,1000,'r3'),(455,NULL,NULL,'r4'),(456,20,NULL,'r5'),(457,20,2000,'r6'),(458,20,2000,'r7'),(459,10,1000,'r8'),(460,20,5980,'r9'),(461,10,NULL,'r10'),(462,20,1000,'r11'),(463,20,NULL,'r12'),(464,60,2000,'r13'),(465,20,2000,'r14'),(466,30,NULL,'r15'),(467,20,NULL,'r16'),(468,NULL,NULL,'r17'),(469,30,1000,'r18'),(470,NULL,1000,'r19'),(471,20,2000,'r20'),(472,60,NULL,'r21'),(473,10,6149,'r22'),(474,10,NULL,'r23'),(475,10,6175,'r24'),(476,30,NULL,'r25'),(477,60,NULL,'r26'),(478,10,2000,'r27'),(479,NULL,1000,'r28'),(480,30,2000,'r29'),(481,10,6253,'r30'),(482,20,6266,'r31'),(483,10,2000,'r32'),(484,10,1000,'r33'),(485,20,6305,'r34'),(486,10,1000,'r35'),(487,20,2000,'r36'),(488,60,6344,'r37'),(489,30,6357,'r38'),(490,20,6370,'r39'),(491,10,6383,'r40'),(492,60,NULL,'r0'),(493,10,NULL,'r1'),(494,20,6422,'r2'),(495,NULL,2000,'r3'),(496,30,1000,'r4'),(497,30,NULL,'r5'),(498,10,6474,'r6'),(499,20,2000,'r7'),(500,10,6500,'r8');
ANALYZE TABLE ta, tb;

SET SESSION tidb_enable_index_merge_join = ON;
-- run the following query ~10 times on unchanged data:
SELECT ta.c0 FROM ta WHERE ta.c0 IN (SELECT c0 FROM tb) GROUP BY ta.c0 ORDER BY ta.c0;
```

**Expected result:** `(10, 20, 30)` — stable across runs (ta.c0 values 10/20/30 all
appear in tb.c0). This is what the default configuration (`tidb_enable_index_merge_join=OFF`,
the v8.5 default) returns, 10/10 runs.

**Actual result:** With `tidb_enable_index_merge_join = ON`, the same query returns a
**random subset** across identical re-runs, e.g. from one session of 10 runs:
`(20,30), (10,30), (20,30), (20,30), (30), (30), (10,20,30), (20,30), (10,20,30), (30)`
— rows are silently dropped, ~80-90% of runs are wrong. No error is raised.

**Key observations (triage):**

1. `EXPLAIN` is byte-identical between correct and wrong runs — the plan is
deterministic; the defect is in execution:
```
└─IndexMergeJoin_31 inner join, inner:HashAgg_28,
outer key:ta.c0, inner key:tb.c0 (Concurrency:5)
├─TableReader(Build): Selection not(isnull(ta.c0)) over TableFullScan ta
└─HashAgg(Probe) over IndexRangeScan tb, range decided by eq(tb.c0, ta.c0)
```
2. `EXPLAIN ANALYZE` on two runs with the same plan shows the IndexMergeJoin
actual output row count flapping (1 vs 3) with fixed input (8 rows), no
concurrent writes.
3. Trigger surface (measured):
- affected: `IN (subquery)` shapes that get the join+count rewrite
(`tidb_opt_insubq_to_join_and_agg`), with or without an outer derived table;
- not affected: `EXISTS` / `NOT IN ... IS NOT NULL` (Apply/anti-join paths);
`tidb_opt_insubq_to_join_and_agg=OFF` + IMJ ON; IMJ OFF (default).
- cost threshold: the IndexMergeJoin operator is only selected when the inner
side is ≳500 rows with an index on the correlation column (≤250 rows did
not select it and did not exhibit the issue).
4. Cross-check: MySQL 8.x-compatible semantics — MySQL 9.7.2 on identical data
returns `(10,20,30)` 6/6 runs.

**Significance:** silent wrong results + nondeterminism, gated behind the new
IndexMergeJoin physical operator (re-introduced v8.4+, default OFF in v8.5).
Any workload or test harness that enables this flag will get randomly wrong
query results without any error signal.

**Environment:** single-node TiDB (docker), no concurrent load, TiUP-free
minimal setup. Reproduced across separate sessions/connections and on a second
freshly initialized instance.

Start by running the reproducible SQL with tidb_enable_index_merge_join enabled and compare repeated results with EXPLAIN ANALYZE. Trace the IndexMergeJoin execution path and the IN-subquery join-and-aggregation rewrite, then add a regression test that repeats the query and verifies the stable (10,20,30) result without concurrent writes.

Written by the indexing model from the issue text.