fix: log prefill parse errors + clean up cron scheduler tests

Follow-up to PR #716 (0xbyt4):
- Log the third remaining silent except-pass in scheduler (prefill
  messages JSON parse failure)
- Fix test mock: run → run_conversation (matches actual agent API)
- Remove unused imports (asyncio, AsyncMock)
- Add test for prefill_messages parse failure logging

cron/scheduler.py

@@ -219,7 +219,8 @@ def run_job(job: dict) -> tuple[bool, str, str, Optional[str]]:
                         prefill_messages = _json.load(_pf)
                     if not isinstance(prefill_messages, list):
                         prefill_messages = None
-                except Exception:
+                except Exception as e:
+                    logger.warning("Job '%s': failed to parse prefill messages file '%s': %s", job_id, pfpath, e)
                     prefill_messages = None
 
         # Max iterations