test(contrib/drivers/gaussdb): add union, DO, raw where test coverage (gogf#4687)

## Summary
- Port 2 test files, 1 testdata SQL, and append Test_Raw_Where from
PgSQL driver to GaussDB driver
- Add `gaussdb_z_unit_feature_union_test.go` (4 tests): Union/UnionAll
query operations
- Add `gaussdb_z_unit_feature_model_do_test.go` (10 tests): DO
struct-based CRUD operations
- Append `Test_Raw_Where` to `gaussdb_z_unit_raw_test.go` (1 test): raw
SQL in Where with subquery and column comparison
- Add `testdata/table_with_prefix.sql` for DO prefix tests
- **15 new test functions**, ~605 net new lines

## Test plan
- [x] `go build ./...` passes
- [x] `gofmt` and `gci` applied
- [x] No remaining `pgsql` references in new files
- [ ] Run full test suite against GaussDB instance

ref gogf#4689

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Co-authored-by: John Guo <claymore1986@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people