fix: resolve CodeQL py/unreachable-statement alert

- Initialize structured_output before retry loop in parallel_orchestrator_reviewer.py
- Fix abstract method __call__ in merge/progress.py (missing pass statement)
- Add MagicMock import to test_integration_phase4.py

apps/backend/merge/progress.py

@@ -66,7 +66,9 @@ class MergeProgressCallback(Protocol):
         percent: int,
         message: str,
         details: dict[str, Any] | None = None,
-    ) -> None: ...
+    ) -> None:
+        """no-op (abstract method)"""
+        pass
 
 
 def emit_progress(

apps/backend/runners/github/services/parallel_orchestrator_reviewer.py

@@ -1731,6 +1731,7 @@ For EACH finding above:
 
         # Retry loop for transient API errors
         last_error = None
+        structured_output = None
         for attempt in range(MAX_VALIDATION_RETRIES + 1):
             if attempt > 0:
                 logger.info(