test(mysql): enhance transaction test coverage with 27 new test functions

Add comprehensive transaction tests covering:
- Isolation levels (READ_COMMITTED, SERIALIZABLE, REPEATABLE_READ):
  * Non-repeatable read scenarios
  * Phantom read prevention
  * Consistent snapshot verification

- Deadlock detection and retry:
  * Two-table deadlock scenarios
  * Same-table deadlock scenarios
  * Automatic retry mechanism

- Deep nested transactions:
  * 7-level nesting with full commit
  * 7-level nesting with partial rollback
  * 10-level maximum depth testing

- Savepoint enhancements:
  * Multiple savepoints in single transaction
  * Rollback to non-existent savepoint error handling

- Concurrent operations:
  * Concurrent inserts (10 goroutines)
  * Concurrent updates to same record

- Mixed propagation modes:
  * REQUIRES_NEW + NESTED + REQUIRED in single flow

- Transaction lifecycle:
  * Rollback after commit (error)
  * Commit after rollback (error)
  * Operations after commit/rollback (error)

- Context integration:
  * Timeout handling
  * Cancellation handling

- Edge cases:
  * Empty transaction with no operations
  * Large batch insert (1000 records)
  * Large batch update (500 records)

- Read-only transactions:
  * Update in read-only transaction (error)
  * Delete in read-only transaction (error)

ref gogf#4689

Author: lingcoder