test: add regression test for stale PID in gateway_state.json (#1631)

Verifies that write_runtime_status() overwrites pid and start_time
from a previous process rather than preserving them via setdefault().
Covers the fix from PR #1632.

tests/gateway/test_status.py

@@ -44,6 +44,26 @@ class TestGatewayPidState:
 
 
 class TestGatewayRuntimeStatus:
+    def test_write_runtime_status_overwrites_stale_pid_on_restart(self, tmp_path, monkeypatch):
+        """Regression: setdefault() preserved stale PID from previous process (#1631)."""
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+
+        # Simulate a previous gateway run that left a state file with a stale PID
+        state_path = tmp_path / "gateway_state.json"
+        state_path.write_text(json.dumps({
+            "pid": 99999,
+            "start_time": 1000.0,
+            "kind": "hermes-gateway",
+            "platforms": {},
+            "updated_at": "2025-01-01T00:00:00Z",
+        }))
+
+        status.write_runtime_status(gateway_state="running")
+
+        payload = status.read_runtime_status()
+        assert payload["pid"] == os.getpid(), "PID should be overwritten, not preserved via setdefault"
+        assert payload["start_time"] != 1000.0, "start_time should be overwritten on restart"
+
     def test_write_runtime_status_records_platform_failure(self, tmp_path, monkeypatch):
         monkeypatch.setenv("HERMES_HOME", str(tmp_path))