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>

run_agent.py

@@ -8793,8 +8793,17 @@ class AIAgent:
             "image/jpg": ".jpg",
         }.get(mime, ".jpg")
         tmp = tempfile.NamedTemporaryFile(prefix="anthropic_image_", suffix=suffix, delete=False)
-        with tmp:
-            tmp.write(base64.b64decode(data))
+        try:
+            with tmp:
+                tmp.write(base64.b64decode(data))
+        except Exception:
+            # delete=False means a corrupt/unsupported data URL would otherwise
+            # leak a zero-byte temp file on every failed materialization.
+            try:
+                os.unlink(tmp.name)
+            except OSError:
+                pass
+            raise
         path = Path(tmp.name)
         return str(path), path