test(backend): add comprehensive tests for OAuth and agent endpoints

- Added tests for OAuth provider admin and consent endpoints covering edge cases.
- Extended agent-related tests to handle incorrect project associations and lifecycle state transitions.
- Introduced tests for sprint status transitions and validation checks.
- Improved multiline formatting consistency across all test functions.

backend/tests/tasks/test_celery_config.py

@@ -10,7 +10,6 @@ These tests verify:
 """
 
 
-
 class TestCeleryAppConfiguration:
     """Tests for the Celery application instance configuration."""