fix: _extract_skill_json crashes on a truthy non-string teacher response (#1630)

src/teacher_escalation.py

@@ -327,7 +327,7 @@ def _extract_skill_json(teacher_response: str) -> Optional[Dict[str, Any]]:
     treated as "teacher declined to write a skill", per the prompt
     contract.
     """
-    if not teacher_response:
+    if not isinstance(teacher_response, str) or not teacher_response:
         return None
     import json
     m = re.search(r"```(?:json)?\s*\n(\{[\s\S]*?\})\s*\n```", teacher_response)