test(spp_studio_api_v2): add comprehensive tests to reach 95% coverage

Add test_coverage_gaps.py with 60+ tests covering monkey-patched service
closures, studio field registration edge cases, extension write service
conversions, variable value service edge cases, and FastAPI endpoint
integration. Coverage: spp_studio_api_v2 95%, spp_gis_report_programs 97%.

Author: emjay0921

spp_studio_api_v2/tests/__init__.py

@@ -4,3 +4,4 @@
 from . import test_studio_field_registration
 from . import test_studio_router
 from . import test_extension_api_integration
+from . import test_coverage_gaps