fix(misc): three small defensive fixes from PR #1974

Salvages the three substantive low-severity fixes from Gutslabs' #1974
"misc bug fixes" bundle.  The other 8 claims in that PR were either
already fixed on main with superior implementations (state lock,
firecrawl lazy import, fcntl/msvcrt guard, path normalization, schema
migrations) or did not survive review.

- run_agent: `_materialize_data_url_for_vision` uses
  `NamedTemporaryFile(delete=False)`; if `base64.b64decode` raises on a
  corrupt data URL the temp file would persist forever.  Wrap the
  write in try/except and `os.unlink` the temp on failure.

- gateway/session: `append_to_transcript` JSONL write had no error
  handling, so disk-full / read-only-fs / permission errors crashed the
  message handler.  The SQLite write above is the primary store, so
  swallow OSError on the JSONL fallback with a debug log.

- gateway/status: `_read_pid_record` reads `pid_path.read_text()` after
  an `exists()` check; if the PID file is deleted between the two
  calls (concurrent gateway restart) we hit an unhandled OSError.
  Catch it and return None.

Adds a regression test for the tempfile cleanup; the other two paths
are defensive try/excepts on infrequent OSError that don't warrant
dedicated tests.

Co-authored-by: Teknium <127238744+teknium1@users.noreply.github.com>

gateway/status.py

@@ -218,7 +218,11 @@ def _read_pid_record(pid_path: Optional[Path] = None) -> Optional[dict]:
     if not pid_path.exists():
         return None
 
-    raw = pid_path.read_text().strip()
+    try:
+        raw = pid_path.read_text().strip()
+    except OSError:
+        # File was deleted between exists() and read_text(), or permission flipped.
+        return None
     if not raw:
         return None