ci: investigate and re-enable 3 excluded test files

[dtsong]

Problem

Both CI workflows (ci.yml:53-56, ci_full.yml:44-47) and the Makefile silently --ignore three test files:

1. test_database_types.py (818 lines) — the most comprehensive type-coverage test exercising every driver's type handling. For a cross-database diff tool, this is arguably the most important test file.
2. test_main.py (249 lines) — tests the CLI entry point
3. test_dbt_config_validators.py (33 lines) — likely uses Pydantic v1 parse_obj API

No comment explains why these are excluded. CI shows green while skipping critical coverage.

Scope

1. Run each excluded test file locally to identify failures
2. Fix the failures (likely Pydantic v1→v2 migration for validators, possible import issues for others)
3. Re-enable in CI, or add explicit pytest.mark.skip with documented reason
4. Remove --ignore flags from ci.yml, ci_full.yml, and Makefile

Key Files

• .github/workflows/ci.yml:53-56
• .github/workflows/ci_full.yml:44-47
• Makefile
• tests/test_database_types.py
• tests/test_main.py
• tests/test_dbt_config_validators.py

Acceptance Criteria

• All three test files either pass in CI or have documented skip reasons
• --ignore flags removed from CI workflows and Makefile
• CI remains green