Inconsistent Query Results Over Time Due to Index Corruption or Cache Issue with BLOB/TEXT Column Comparison

## Bug Report

This bug reveals a data inconsistency where identical queries return different results (0 vs 1 ) after a time delay. This may be unstable index behavior or caching issues involving BLOB/TEXT column operations, partial indexing, and CAST conversions.

### 1. Minimal reproduce step (Required)

```sql
CREATE TABLE t0(c0 BLOB, c1 TEXT);
CREATE TABLE t1 LIKE t0;
INSERT INTO t0 VALUES ('a', ''), ('b', ' ');
UPDATE t0 SET c0=CAST(t0.c1 AS BINARY), c1='c' WHERE true;
CREATE INDEX i0 ON t1(c0(1));
INSERT INTO t1 VALUES ('', '');
SELECT COUNT(CASE WHEN t0.c0 != t1.c0 THEN 1 END) FROM t0 INNER JOIN t1 ON t0.c0 = t1.c0;
+--------------------------------------------+
| COUNT(CASE WHEN t0.c0 != t1.c0 THEN 1 END) |
+--------------------------------------------+
| 0 |
+--------------------------------------------+

-- wait 2 minutes and execute it again
SELECT COUNT(CASE WHEN t0.c0 != t1.c0 THEN 1 END) FROM t0 INNER JOIN t1 ON t0.c0 = t1.c0;
+--------------------------------------------+
| COUNT(CASE WHEN t0.c0 != t1.c0 THEN 1 END) |
+--------------------------------------------+
| 1 |
+--------------------------------------------+
```

### 2. What did you expect to see? (Required)
```sql
SELECT COUNT(CASE WHEN t0.c0 != t1.c0 THEN 1 END) FROM t0 INNER JOIN t1 ON t0.c0 = t1.c0;
+--------------------------------------------+
| COUNT(CASE WHEN t0.c0 != t1.c0 THEN 1 END) |
+--------------------------------------------+
| 0 |
+--------------------------------------------+
```
### 3. What did you see instead (Required)
```sql
SELECT COUNT(CASE WHEN t0.c0 != t1.c0 THEN 1 END) FROM t0 INNER JOIN t1 ON t0.c0 = t1.c0;
+--------------------------------------------+
| COUNT(CASE WHEN t0.c0 != t1.c0 THEN 1 END) |
+--------------------------------------------+
| 1 |
+--------------------------------------------+
```
### 4. What is your TiDB version? (Required)

| Release Version: v8.5.4
Edition: Community
Git Commit Hash: e4e814fdc0afe9c3a6e5e96f129d83df802ab820
Git Branch: HEAD
UTC Build Time: 2025-11-26 15:53:39
GoVersion: go1.23.12
Race Enabled: false
Check Table Before Drop: false
Store: tikv |