CAST(x AS YEAR) skips the two-digit-year conversion used by column inserts

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
SELECT CAST(70 AS YEAR); -- 70
CREATE TABLE y(y YEAR);
INSERT INTO y VALUES (70);
SELECT y+0 FROM y; -- 1970 (70–99 → 1900s per MySQL rules)
```

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

```
mysql>
mysql> SELECT CAST(70 AS YEAR); -- 70
+------------------+
| CAST(70 AS YEAR) |
+------------------+
| 1970 |
+------------------+
1 row in set (0.00 sec)

mysql> CREATE TABLE y(y YEAR);
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO y VALUES (70);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT y+0 FROM y; -- 1970 (70–99 → 1900s per MySQL rules)
+------+
| y+0 |
+------+
| 1970 |
+------+
1 row in set (0.00 sec)

mysql> select version();
+-----------+
| version() |
+-----------+
| 9.7.2 |
+-----------+
1 row in set (0.00 sec)
```

### 3. What did you see instead (Required)
```
mysql>
mysql> SELECT CAST(70 AS YEAR); -- 70
+------------------+
| CAST(70 AS YEAR) |
+------------------+
| 70 |
+------------------+
1 row in set (0.00 sec)

mysql> CREATE TABLE y(y YEAR);
Query OK, 0 rows affected, 1 warning (0.01 sec)

mysql> INSERT INTO y VALUES (70);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT y+0 FROM y; -- 1970 (70–99 → 1900s per MySQL rules)
+------+
| y+0 |
+------+
| 1970 |
+------+
1 row in set (0.00 sec)

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

```
mysql> select tidb_version();
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v9.0.0-beta.2.pre-2174-g65ac2fad58-dirty
Edition: Community
Git Commit Hash: 65ac2fad582510b92d3c4b79999e48217c989737
Git Branch: master
UTC Build Time: 2026-08-30 15:18:00
GoVersion: go1.25.12
Race Enabled: false
Check Table Before Drop: false
Store: unistore
Kernel Type: Classic |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
```

Start by reproducing the reported SELECT CAST(70 AS YEAR) and INSERT behavior in TiDB, then trace the separate CAST and YEAR column conversion paths. Compare their handling of two-digit years and add a regression test covering the query result; done means CAST(70 AS YEAR) matches the column-insert conversion without breaking the existing cases.

Written by the indexing model from the issue text.