information_schema.tidb_statements_stats allows unauthorized data access

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
-- connect as root
CREATE USER foo;

-- connect as foo
SELECT SAMPLE_USER, DIGEST_TEXT, QUERY_SAMPLE_TEXT, EXEC_COUNT
FROM information_schema.tidb_statements_stats
WHERE SAMPLE_USER='root';
```
### 2. What did you expect to see? (Required)
```
MySQL [(none)]> SELECT SAMPLE_USER, DIGEST_TEXT, QUERY_SAMPLE_TEXT, EXEC_COUNT
-> FROM information_schema.tidb_statements_stats
-> WHERE SAMPLE_USER='root';
Empty set (0.001 sec)
```
### 3. What did you see instead (Required)
```
MySQL [(none)]> SELECT SAMPLE_USER, DIGEST_TEXT, QUERY_SAMPLE_TEXT, EXEC_COUNT
-> FROM information_schema.tidb_statements_stats
-> WHERE SAMPLE_USER='root';
+-------------+----------------------------------+----------------------------------+------------+
| SAMPLE_USER | DIGEST_TEXT | QUERY_SAMPLE_TEXT | EXEC_COUNT |
+-------------+----------------------------------+----------------------------------+------------+
| root | select @@version_comment limit ? | select @@version_comment limit 1 | 2 |
+-------------+----------------------------------+----------------------------------+------------+
1 row in set (0.001 sec)
```
### 4. What is your TiDB version? (Required)

```
Release Version: v9.0.0-beta.1
Edition: Community
Git Commit Hash: 7aff918dcbfa6facf2adef9ade9961c40f217421
Git Branch: HEAD
UTC Build Time: 2025-03-24 09:09:55
GoVersion: go1.23.7
Race Enabled: false
Check Table Before Drop: false
Store: unistore
```