GBK in TiDB is incompatible with that in MySQL

## Bug Report

### 1. Minimal reproduce step (Required)

```sql
SELECT CONVERT(CAST(0xA2E3 AS CHAR CHARACTER SET gbk) USING utf8mb4);

SELECT CONVERT(CAST(0xA2E4 AS CHAR CHARACTER SET gbk) USING utf8mb4);
```

### 2. What did you expect to see? (Required)

(If it is fully compatible with MySQL)

Both should be:

```
+---------------------------------------------------------------+
| CONVERT(CAST(0xA2E3 AS CHAR CHARACTER SET gbk) USING utf8mb4) |
+---------------------------------------------------------------+
| ? |
+---------------------------------------------------------------+
```

### 3. What did you see instead (Required)

The first one:

```
+---------------------------------------------------------------+
| CONVERT(CAST(0xA2E3 AS CHAR CHARACTER SET gbk) USING utf8mb4) |
+---------------------------------------------------------------+
| € |
+---------------------------------------------------------------+
1 row in set (0.001 sec)
```

The second one (with warning):

```
+---------------------------------------------------------------+
| CONVERT(CAST(0xA2E4 AS CHAR CHARACTER SET gbk) USING utf8mb4) |
+---------------------------------------------------------------+
| NULL |
+---------------------------------------------------------------+
1 row in set, 1 warning (0.001 sec)

MySQL [demo_tidb]> SHOW WARNINGS;
+---------+------+-----------------------------------------------------+
| Level | Code | Message |
+---------+------+-----------------------------------------------------+
| Warning | 3854 | Cannot convert string '\xA2\xE4' from binary to gbk |
+---------+------+-----------------------------------------------------+
1 row in set (0.001 sec)
```

### 4. What is your TiDB version? (Required)

```
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v8.5.0
Edition: Community
Git Commit Hash: d13e52ed6e22cc5789bed7c64c861578cd2ed55b
Git Branch: HEAD
UTC Build Time: 2024-12-18 02:26:06
GoVersion: go1.23.3
Race Enabled: false
Check Table Before Drop: false
Store: unistore |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
```

## More Info

In TiDB,

```
> SELECT @@sql_mode, @@character_set_client, @@character_set_connection, @@character_set_results;
+-------------------------------------------------------------------------------------------------------------------------------------------+------------------------+----------------------------+-------------------------+
| @@sql_mode | @@character_set_client | @@character_set_connection | @@character_set_results |
+-------------------------------------------------------------------------------------------------------------------------------------------+------------------------+----------------------------+-------------------------+
| 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 | utf8mb4 | utf8mb4 | utf8mb4 |
+-------------------------------------------------------------------------------------------------------------------------------------------+------------------------+----------------------------+-------------------------+
```

In MySQL,

```
+-----------------------------------------------------------------------------------------------------------------------+------------------------+----------------------------+-------------------------+
| @@sql_mode | @@character_set_client | @@character_set_connection | @@character_set_results |
+-----------------------------------------------------------------------------------------------------------------------+------------------------+----------------------------+-------------------------+
| ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION | utf8mb4 | utf8mb4 | utf8mb4 |
+-----------------------------------------------------------------------------------------------------------------------+------------------------+----------------------------+-------------------------+
```

In fact, there many such divergences,

Summary:
- mismatched mappings: 82
- present only in mysql_valid_gbk_utf8_list.txt: 2067
- present only in tidb_valid_gbk.txt: 0

I know that there is no *single* standard in terms of character set mappings, but TiDB declares that it is compatible with MySQL. The divergences are not mentioned in [character-set-gbk](https://docs.pingcap.com/tidb/stable/character-set-gbk/). So it can be seen as bugs.

Start by reproducing the two GBK conversion queries and compare their results with MySQL. Then inspect the GBK mapping data, including mysql_valid_gbk_utf8_list.txt and tidb_valid_gbk.txt, to identify the compatibility gaps; done means the reported mappings match MySQL and the divergences are covered by regression tests or documented.

Written by the indexing model from the issue text.