Add unit tests for mail account endpoints (30 tests)

Comprehensive test coverage for backend/app/api/v1/endpoints/mail_accounts.py:
- CRUD operations (create, list, get, update, delete)
- Subscription limit enforcement and superuser bypass
- Account toggle with ERROR→ACTIVE status reset
- Pull-now with disabled account guard
- Test connection (new and existing) with decrypt failure handling
- Auto-detect mail settings
- Paginated processing runs and logs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Unit tests for mail account endpoints (`test_mail_accounts.py`): 30 tests covering CRUD, toggle, pull-now, test connection, auto-detect, processing runs, and processing logs
 - Unit tests for authentication endpoints (`test_auth.py`): 22 tests covering register, login, Google OAuth, authorize-url, and helper functions
 
 ## v0.6.0 (2026-03-29)