test: increase coverage for imap_tasks.py and fix config_validator.py

- Add comprehensive tests for imap_tasks.py covering:
  - Lock acquire/release mechanisms
  - pull_all_inboxes task with various scenarios
  - pull_inbox for Gmail and non-Gmail with edge cases
  - find_all_mail_xlist functionality
  - Extended fetch_attachments tests for all MIME types
  - Edge cases: invalid JSON, missing Message-ID, already processed, etc.
- Achieve 98.26% coverage for imap_tasks.py (up from 48.78%)

- Fix config_validator.py to include validate_auth_config export
- Update tests to verify all exports including validate_auth_config
- Note: config_validator.py file is shadowed by config_validator/ directory
  in Python's module resolution, so it cannot be directly imported or tested.
  The package's __init__.py (which has 100% coverage) is what's actually used.

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

app/utils/config_validator.py

@@ -11,6 +11,7 @@ from app.utils.config_validator.settings_display import dump_all_settings, get_s
 # Import and re-export all functions from the new package
 from app.utils.config_validator.validators import (
     check_all_configs,
+    validate_auth_config,
     validate_email_config,
     validate_notification_config,
     validate_storage_configs,
@@ -20,6 +21,7 @@ __all__ = [
     "validate_email_config",
     "validate_storage_configs",
     "validate_notification_config",
+    "validate_auth_config",
     "mask_sensitive_value",
     "get_provider_status",
     "get_settings_for_display",