test(notification-service): cover payload cms validation and seeding

[romdj]

The SonarQube quality gate on main failed with 64.2% coverage on new code against the required 80%. The uncovered changes were concentrated in the notification service's Payload CMS migration.

Add 51 tests covering collection validation and audit hooks, environment configuration, and template seeding on empty/existing databases and error paths. The tests exercise the real collection hooks and seed entry point while mocking the external CMS boundary. No production code or coverage exclusions change.

Validation:

• Full notification coverage suite: 143 passed, 2 existing skips; line coverage increased from 34.1% to 81.79%.
• All five targeted CMS files have 100% local line coverage, including every CMS new-code line Sonar marked uncovered.
• Repository lint, type checks, and service unit suites passed on Node 24; three existing frontend type-check warnings remain.
• New tests also passed a dedicated TypeScript check and ESLint.

SonarQube's new-code gate still needs confirmation in CI; the local service line-coverage percentage is a separate metric.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud