test(contrib/drivers/mysql): enhance transaction tests (gogf#4704)

## Summary
- Add nested transaction tests
- Add transaction propagation tests
- Add transaction rollback/commit behavior tests
- Add transaction context handling tests

**Test coverage added:** ~25 test functions

Ref gogf#4689

## Test plan
```bash
cd contrib/drivers/mysql
go test -v -run "TestTX_Nested|TestTX_Propagation|TestTX_Transaction"
```

Author: lingcoder