fix: add missing test comment for backend commenting standard (Task 2 review)

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

backend/tests/observability/test_tracked_client.py

@@ -11,6 +11,8 @@ from app.shared.model_usage_tracker import ModelUsageTracker
 
 def _make_inner(model: str = "deepseek-v4-flash") -> MagicMock:
     """Build a MagicMock standing in for a concrete BaseLLMClient subclass."""
+    # Use MagicMock to avoid requiring a real LLM provider implementation (e.g., DeepseekClient);
+    # tests focus on TrackedLLMClient's delegation and recording behavior, not provider logic.
     inner = MagicMock()
     inner.config = LLMConfig(
         provider=LLMProvider.DEEPSEEK, model=model, api_key="test-key", base_url="http://example.test/v1",