fix(gateway): also unlink stale PID + lock files on cleanup

Follow-up for salvaged PR #14179.

`_cleanup_invalid_pid_path` previously called `remove_pid_file()` for the
default PID path, but that helper defensively refuses to delete a PID file
whose pid field differs from `os.getpid()` (to protect --replace handoffs).
Every realistic stale-PID scenario is exactly that case: a crashed/Ctrl+C'd
gateway left behind a PID file owned by a now-dead foreign PID.

Once `get_running_pid()` has confirmed the runtime lock is inactive, the
on-disk metadata is known to belong to a dead process, so we can force-unlink
both the PID file and the sibling `gateway.lock` directly instead of going
through the defensive helper.

Also adds a regression test with a dead foreign PID that would have failed
against the previous cleanup logic.

gateway/status.py

@@ -241,13 +241,23 @@ def _pid_from_record(record: Optional[dict[str, Any]]) -> Optional[int]:
 
 
 def _cleanup_invalid_pid_path(pid_path: Path, *, cleanup_stale: bool) -> None:
+    """Delete a stale gateway PID file (and its sibling lock metadata).
+
+    Called from ``get_running_pid()`` after the runtime lock has already been
+    confirmed inactive, so the on-disk metadata is known to belong to a dead
+    process.  Unlike ``remove_pid_file()`` (which defensively refuses to delete
+    a PID file whose ``pid`` field differs from ``os.getpid()`` to protect
+    ``--replace`` handoffs), this path force-unlinks both files so the next
+    startup sees a clean slate.
+    """
     if not cleanup_stale:
         return
     try:
-        if pid_path == _get_pid_path():
-            remove_pid_file()
-        else:
-            pid_path.unlink(missing_ok=True)
+        pid_path.unlink(missing_ok=True)
+    except Exception:
+        pass
+    try:
+        _get_gateway_lock_path(pid_path).unlink(missing_ok=True)
     except Exception:
         pass