fix(gateway): handle Windows stop fallback

nanobot/gateway/runtime.py

@@ -293,14 +293,26 @@ class GatewayRuntime:
     def _terminate_windows(self, pid: int, *, timeout_s: int) -> bool:
         ctrl_break = getattr(signal, "CTRL_BREAK_EVENT", None)
         if ctrl_break is not None:
-            with suppress(ProcessLookupError):
+            # Detached Windows children can reject CTRL_BREAK_EVENT with WinError 87;
+            # keep the existing taskkill fallback for that process shape.
+            with suppress(ProcessLookupError, OSError):
                 os.kill(pid, ctrl_break)
             if self._wait_for_exit(pid, timeout_s):
                 return True
-        self._subprocess_run(["taskkill", "/PID", str(pid), "/T"], check=False)
+        self._subprocess_run(
+            ["taskkill", "/PID", str(pid), "/T"],
+            check=False,
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.DEVNULL,
+        )
         if self._wait_for_exit(pid, 2):
             return True
-        self._subprocess_run(["taskkill", "/PID", str(pid), "/T", "/F"], check=False)
+        self._subprocess_run(
+            ["taskkill", "/PID", str(pid), "/T", "/F"],
+            check=False,
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.DEVNULL,
+        )
         return self._wait_for_exit(pid, 2)
 
     def _wait_for_exit(self, pid: int, timeout_s: int | float) -> bool:

tests/gateway/test_runtime.py

@@ -1,4 +1,6 @@
 import json
+import signal
+import subprocess
 from pathlib import Path
 
 from nanobot.gateway import GatewayRuntime, GatewayRuntimePaths, GatewayStartOptions
@@ -168,3 +170,42 @@ def test_stop_keeps_state_when_process_survives_timeout(tmp_path, monkeypatch):
     assert result.status.running is True
     assert result.status.reason == "stop_timeout"
     assert runtime.paths.state_path.exists()
+
+
+def test_terminate_windows_falls_back_when_ctrl_break_is_rejected(tmp_path, monkeypatch):
+    taskkill_calls: list[dict] = []
+
+    def fake_run(command, **kwargs):
+        taskkill_calls.append({"command": command, "kwargs": kwargs})
+
+    runtime = GatewayRuntime(
+        paths=_paths(tmp_path),
+        platform_name="Windows",
+        subprocess_run=fake_run,
+        sleep=lambda _seconds: None,
+    )
+
+    monkeypatch.setattr(signal, "CTRL_BREAK_EVENT", 1, raising=False)
+
+    def fake_kill(_pid, _signal):
+        raise OSError(87, "The parameter is incorrect")
+
+    monkeypatch.setattr("nanobot.gateway.runtime.os.kill", fake_kill)
+
+    def fake_wait_for_exit(_pid, _timeout_s):
+        # Simulate a process that only exits after the taskkill fallback runs.
+        return bool(taskkill_calls)
+
+    monkeypatch.setattr(runtime, "_wait_for_exit", fake_wait_for_exit)
+
+    assert runtime._terminate_windows(12345, timeout_s=20) is True
+    assert taskkill_calls == [
+        {
+            "command": ["taskkill", "/PID", "12345", "/T"],
+            "kwargs": {
+                "check": False,
+                "stdout": subprocess.DEVNULL,
+                "stderr": subprocess.DEVNULL,
+            },
+        }
+    ]