TopN Push-down Incorrect String Comparison for TEXT Column Leads to Wrong `MIN()` Result

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal Reproduce Step (Required)

```sql
DROP DATABASE IF EXISTS repro_tidb626_db3;
CREATE DATABASE repro_tidb626_db3 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
USE repro_tidb626_db3;

SET SESSION sql_mode='ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

CREATE TABLE src (
vp_rowid BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
c0 TEXT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

INSERT INTO src(c0) VALUES (' '), (''), ('y');

CREATE TABLE l (vp_rowid BIGINT NOT NULL PRIMARY KEY) ENGINE=InnoDB;
CREATE TABLE r (
vp_rowid BIGINT NOT NULL PRIMARY KEY,
c0 TEXT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

INSERT INTO l SELECT vp_rowid FROM src;
INSERT INTO r SELECT vp_rowid, c0 FROM src;

CREATE VIEW v AS
SELECT l.vp_rowid, r.c0
FROM l JOIN r ON l.vp_rowid = r.vp_rowid;

-- Single‑table query (wrong: returns space ' ')
SELECT 'single' AS q, CONCAT('[', MIN(c0), ']') AS shown, HEX(MIN(c0)) AS hex, LENGTH(MIN(c0)) AS len
FROM src;

-- View query (correct: returns empty string)
SELECT 'split' AS q, CONCAT('[', MIN(c0), ']') AS shown, HEX(MIN(c0)) AS hex, LENGTH(MIN(c0)) AS len
FROM v;
```

### 2. What Did You Expect to See? (Required)

String comparison follows the collation: an empty string `''` is smaller than a space `' '`. Thus `MIN(c0)` should return `''` (empty string) for both the single‑table and the view query. All results should be identical.

### 3. What Did You See Instead? (Required)

- Single‑table query (default): returns `' '` (space, HEX `20`, length `1`) — wrong.
- View query: returns `''` (empty string, HEX `''`, length `0`) — correct.

### 4. What Is Your TiDB Version? (Required)

Version: TiDB‑v9.0.0

### 5. Execution Plan Differences

**Single‑table `MIN(c0)` (wrong)** — TopN pushed down to TiKV:

```
StreamAgg_13 root funcs:min(c0)
└─TopN_14 root c0, offset:0, count:1
└─TableReader_25
└─TopN_24 cop[tikv] c0, offset:0, count:1
└─Selection_23 not(isnull(c0))
└─TableFullScan_22 table:src
```

**View `MIN(c0)` (correct)** — TopN stays at root:

```
StreamAgg_21 root funcs:min(r.c0)
└─TopN_24 root r.c0, offset:0, count:1
└─MergeJoin_32 inner join ...
```

**`AGG_TO_COP` hint (correct)** — Uses cop aggregation instead of TopN:
The plan shows a similar shape but the push‑down operates as a traditional aggregate, not a TopN operator.

### 6. Root Cause

TiDB's optimizer converts `MIN(c0)` into a `TopN(c0, 1)` (i.e., `ORDER BY c0 LIMIT 1`) and may push this TopN operator down to the TiKV coprocessor for single‑table scans. Inside TiKV, the TopN execution engine uses its own internal comparison logic for string (`TEXT`/Blob) values. This comparison is buggy: it erroneously treats `' '` (space) as smaller than `''` (empty string), violating the collation order where `''` should be the smallest possible string.

When the TopN remains in the TiDB root layer (as in the view query or when the push‑down is prevented), or when the aggregation is performed via the traditional `AGG_TO_COP` path, the comparison uses TiDB's own expression evaluator or TiKV's aggregate comparator — both of which correctly order `'' < ' '`. Consequently, the correct minimum is returned.

Run the provided SQL reproduction on TiDB v9.0.0 and compare the single-table, view, and AGG_TO_COP plans. Trace the TopN push-down path into the TiKV coprocessor and its TEXT comparison logic; done means MIN(c0) returns the empty string for both queries without changing the correct view or AGG_TO_COP behavior.

Written by the indexing model from the issue text.