Fix panic miss Free in PartitionBy.reset for SUBPARTITION DDL (#23168)

Fix `panic miss Free` in `PartitionBy.reset()` when parsing SUBPARTITION DDL statements.

**Root Cause:**
In `sub_partition_opt`, `sub_partition_method` returns `*PartitionBy`, but we only need the `PType` field. The temporary `*PartitionBy` object must be freed after extracting `PType`, otherwise it leaks and causes panic in `reset()` when the object is recycled.

**Solution:**
- Extract `PType` from `*PartitionBy` before freeing
- Set `PType` to nil to avoid double-free
- Free the temporary `*PartitionBy` object properly

**Changes:**
- `pkg/sql/parsers/dialect/mysql/mysql_sql.y`: Fix `sub_partition_opt` to extract `PType` and free temporary object
- `pkg/sql/parsers/dialect/mysql/mysql_sql.go`: Regenerated parser code

**Testing:**
The fix resolves the panic for SUBPARTITION DDL statements like:
```sql
CREATE TABLE test_subpartition (
id INT,
created_date DATE,
region VARCHAR(50)
) PARTITION BY RANGE (YEAR(created_date))
SUBPARTITION BY HASH(region) (
PARTITION p2022 VALUES LESS THAN (2023) (
SUBPARTITION s1,
SUBPARTITION s2
)
);
```

Approved by: @iamlinjunhong

Author: aptend