fix(cookbook): skip the pid sweep when the host has no procfs

`_cookbook_kill_session` kills the tmux session, then sweeps /proc for
model servers that survived the SIGHUP. The sweep had no guard, so on
macOS and Windows `os.listdir("/proc")` raised FileNotFoundError after
the kill had already succeeded. The function's outer except turned that
into `{"error": "...No such file or directory: '/proc'", "exit_code": 1}`
and skipped the state write that marks the session stopped — the agent
is told a stop failed that actually worked.

Guard the sweep with a procfs check, the way `_scan_running_model_processes`
already does a few hundred lines up. The root and the check now live in
`core/platform_compat`, which is where OS differences belong and which
makes both branches patchable from a test on either kind of host.

Adds a class-level guard test: the third instance of this defect, and two
of the three were found by reading source rather than by a test.

tests/test_cookbook_stop_without_procfs.py

@@ -0,0 +1,166 @@
+"""Stopping a Cookbook server must succeed on a host with no procfs.
+
+The tmux kill is what actually stops the server; the pid sweep that follows
+it only catches model servers that survive the session's SIGHUP. On macOS and
+Windows there is no ``/proc`` to sweep, and letting that raise turned a
+successful stop into a reported failure *and* skipped the state write that
+marks the session stopped for the Cookbook UI.
+"""
+import asyncio
+import json
+import signal
+
+import pytest
+
+from core import platform_compat
+from src import tool_implementations as tools
+
+
+class FakeResponse:
+    def __init__(self, data=None, status_code=200):
+        self._data = data or {}
+        self.status_code = status_code
+        self.text = json.dumps(self._data)
+
+    def json(self):
+        return self._data
+
+
+def _tracked_state(session_id="serve-abc123", cmd="python -m vllm.entrypoints.openai.api_server"):
+    return {
+        "tasks": [
+            {
+                "sessionId": session_id,
+                "model": "org/model",
+                "type": "serve",
+                "status": "running",
+                "payload": {"_cmd": cmd},
+            }
+        ]
+    }
+
+
+def _install_httpx_client(monkeypatch, state):
+    """Serve cookbook state over a fake httpx and record every POST body."""
+    import httpx
+
+    posts = []
+
+    class FakeAsyncClient:
+        def __init__(self, *args, **kwargs):
+            pass
+
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, exc_type, exc, tb):
+            return False
+
+        async def get(self, url, **kwargs):
+            return FakeResponse(state)
+
+        async def post(self, url, json=None, **kwargs):
+            posts.append((url, json))
+            return FakeResponse({"ok": True})
+
+    monkeypatch.setattr(httpx, "AsyncClient", FakeAsyncClient)
+    return posts
+
+
+def _install_successful_tmux_kill(monkeypatch):
+    """Replace the real ``tmux kill-session`` with a process that succeeds."""
+
+    class FakeProc:
+        returncode = 0
+
+        async def communicate(self):
+            return b"", b""
+
+    async def fake_exec(*argv, **kwargs):
+        assert argv[:2] == ("tmux", "kill-session")
+        return FakeProc()
+
+    monkeypatch.setattr(asyncio, "create_subprocess_exec", fake_exec)
+
+
+def _stopped_statuses(posts, session_id):
+    out = []
+    for _url, body in posts:
+        for task in (body or {}).get("tasks") or []:
+            if task.get("sessionId") == session_id:
+                out.append(task.get("status"))
+    return out
+
+
+@pytest.mark.asyncio
+async def test_stop_marks_session_stopped_when_the_host_has_no_procfs(
+    monkeypatch, tmp_path
+):
+    state = _tracked_state()
+    posts = _install_httpx_client(monkeypatch, state)
+    _install_successful_tmux_kill(monkeypatch)
+    monkeypatch.setattr(platform_compat, "PROC_ROOT", tmp_path / "no-procfs")
+
+    import os
+
+    def _unexpected_listdir(*args, **kwargs):
+        raise AssertionError("the pid sweep must not run without procfs")
+
+    monkeypatch.setattr(os, "listdir", _unexpected_listdir)
+
+    result = await tools.do_stop_served_model(
+        json.dumps({"session_id": "serve-abc123"})
+    )
+
+    assert result == {"output": "Stopped server serve-abc123", "exit_code": 0}
+    assert _stopped_statuses(posts, "serve-abc123") == ["stopped"]
+
+
+@pytest.mark.asyncio
+async def test_stop_sweeps_surviving_pids_when_procfs_is_present(
+    monkeypatch, tmp_path
+):
+    tracked_cmd = "python -m vllm.entrypoints.openai.api_server --model org/model"
+    state = _tracked_state(cmd=tracked_cmd)
+    posts = _install_httpx_client(monkeypatch, state)
+    _install_successful_tmux_kill(monkeypatch)
+
+    proc = tmp_path / "proc"
+
+    def _write_pid(pid, cmdline):
+        entry = proc / pid
+        entry.mkdir(parents=True)
+        (entry / "cmdline").write_bytes(cmdline.replace(" ", "\0").encode())
+
+    _write_pid("101", tracked_cmd)
+    _write_pid("202", "python -m http.server")
+    (proc / "self").mkdir()
+    monkeypatch.setattr(platform_compat, "PROC_ROOT", proc)
+
+    signalled = []
+    import os
+
+    monkeypatch.setattr(os, "kill", lambda pid, sig: signalled.append((pid, sig)))
+
+    result = await tools.do_stop_served_model(
+        json.dumps({"session_id": "serve-abc123"})
+    )
+
+    assert result["exit_code"] == 0
+    assert (101, signal.SIGTERM) in signalled
+    assert not any(pid == 202 for pid, _sig in signalled)
+    assert _stopped_statuses(posts, "serve-abc123") == ["stopped"]
+
+
+def test_model_process_scan_returns_empty_without_procfs(monkeypatch, tmp_path):
+    """The other procfs scan in the same module already guards; pin it."""
+    monkeypatch.setattr(platform_compat, "PROC_ROOT", tmp_path / "no-procfs")
+
+    import os
+
+    def _unexpected_listdir(*args, **kwargs):
+        raise AssertionError("the model-process scan must not run without procfs")
+
+    monkeypatch.setattr(os, "listdir", _unexpected_listdir)
+
+    assert tools._scan_running_model_processes() == []