fix(backend): PATCH technique_ids returns 503 when MITRE bundle not loaded

Added bundle-loaded guard in _resolve_technique_ids() before attempting any
lookup; matches behavior of GET /api/mitre/matrix and GET /api/mitre/techniques.
Added corresponding test case.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/simulation_workflow.py

@@ -55,6 +55,9 @@ def _resolve_technique_ids(
     """
     from backend.app.services import mitre as mitre_svc
 
+    if not mitre_svc.mitre_loaded:
+        return None, (jsonify({"error": "mitre bundle not loaded"}), 503)
+
     # Dedup, preserve order.
     seen: dict[str, None] = dict.fromkeys(technique_ids)
     resolved: list[dict[str, str]] = []