Write integration tests for Config Service and Validator

Place integration tests in `integration_test/` (not `test/`) to signal they require external dependencies. Use a `TestSupabaseFactory` helper class that reads SUPABASE_TEST_URL and SUPABASE_TEST_ANON_KEY from the environment and creates pre-authenticated clients for org A and org B test sessions. For the transactional rollback test, the cleanest approach is to inject the audit logger as a dependency into IntegrationConfigService (following the pattern established in task-014) and provide a `ThrowingAuditLogger` test double. For RLS cross-org isolation, create two separate `SupabaseClient` instances with different JWT tokens representing org A and org B — use Supabase's `auth.signInWithPassword()` with pre-seeded test users.

Cache invalidation spy: `class SpyFieldMappingResolver implements FieldMappingResolver { int invalidateCalls = 0; @override Future invalidateCache(String orgId, String integrationType) async { invalidateCalls++; } }`. Avoid using `expect()` inside `setUp` — only use it in test bodies so failures are clearly attributed.

This task IS the testing work. Use `flutter_test` with a `setUpAll` that initializes the Supabase test client using environment variables. Use a `tearDown` that deletes all rows created by the test using a service-role client (to bypass RLS for cleanup only). Group tests as: 'save flow - valid configs', 'save flow - validation rejection', 'upsert deduplication', 'connection test orchestration', 'field mapping cache invalidation', 'transactional rollback', 'RLS cross-org isolation'.

For the cache invalidation test, use a spy wrapper around FieldMappingResolver that records whether invalidateCache() was called and with which arguments. For the rollback test, inject a failing audit logger that always throws — verify the config table has no new row after the attempted save. Run tests with `flutter test integration_test/config_service_test.dart`. Ensure test environment setup instructions are documented in a comment at the top of the test file.