test: expand gmail_service.py unit test coverage (10 new tests)

Add tests for uncovered branches:
- inject_email HttpError handling
- get_or_create_label: existing label, create new, HttpError, generic error
- inject_debug_email full flow + dedup of test label
- get_refreshed_token: token changed vs unchanged
- service property lazy initialization via build()

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
 
+- Expanded `gmail_service.py` unit test coverage: 10 new tests covering `inject_email` HttpError branch, `get_or_create_label` (existing/create/HttpError/generic-error), `inject_debug_email` full flow, `get_refreshed_token` (changed/unchanged), and `service` property lazy initialization — 24 total tests in `test_gmail_service.py`
 - Expanded `api.ts` test coverage from 20% to near-complete: 56 new tests covering all 10 API objects (`authApi`, `userApi`, `mailAccountsApi`, `processingRunsApi`, `gmailApi`, `smtpApi`, `adminApi`, `notificationsApi`, `adminNotificationsApi`, `versionApi`) — 65 total tests in `api.test.ts`
 - Unit tests for `AddMailAccountModal` component (`AddMailAccountModal.test.tsx`): 28 tests covering create/edit mode rendering, provider wizard flow, form field changes, checkbox toggles, auto-detect, test connection, submit mutations, error extraction, and modal close interactions
 - Unit tests for provider endpoints (`test_providers.py`): 23 tests covering provider presets, Gmail credential CRUD, import labels, authorize URL, debug email, and OAuth callback