test(contrib/drivers/pgsql): add Union, DO and Raw Where test coverage (#4679)

## Summary
- Add `pgsql_z_unit_feature_union_test.go`: 4 tests for Union/UnionAll
on both db and model level
- Add `pgsql_z_unit_feature_model_do_test.go`: 10 tests for DO (Data
Object) pattern - insert, batch insert, update, pointer fields, WHERE,
DAO pattern, and field prefix handling
- Enhance `pgsql_z_unit_raw_test.go`: add `Test_Raw_Where` for subquery
NOT EXISTS and field comparison using `gdb.Raw()`, adapted for PgSQL
double-quote quoting
- Add `testdata/table_with_prefix.sql` for PgSQL-compatible FieldPrefix
test

All tests adapted from MySQL driver test suite with PgSQL-specific
adjustments:
- Nullable table schema for DO partial inserts (PgSQL NOT NULL is
stricter than MySQL)
- Double-quote identifier quoting instead of backticks
- Unquoted table aliases in generated SQL

## Test plan
- [x] All 15 new tests pass locally
- [x] Full pgsql test suite (107 tests) passes with zero regressions

ref #4689

---------

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

contrib/drivers/pgsql/testdata/table_with_prefix.sql

@@ -0,0 +1,6 @@
+DROP TABLE IF EXISTS instance;
+CREATE TABLE instance (
+    f_id SERIAL NOT NULL PRIMARY KEY,
+    name varchar(255) DEFAULT ''
+);
+INSERT INTO instance VALUES (1, 'john');