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/crud/syndarix/test_agent_instance.py

@@ -203,7 +203,7 @@ class TestAgentInstanceGetByProject:
         self, db_session, test_project, test_agent_instance
     ):
         """Test getting agent instances with status filter."""
-        instances, total = await agent_instance.get_by_project(
+        instances, _total = await agent_instance.get_by_project(
             db_session,
             project_id=test_project.id,
             status=AgentStatus.IDLE,