fix(mcp): per-process PID isolation prevents cross-session crash on restart

- _stdio_pids: set → Dict[int,str] tracks pid→server_name
- SIGTERM-first with 2s grace before SIGKILL escalation
- hasattr guard for SIGKILL on platforms without it
- Updated tests for dict-based tracking and 3-phase kill sequence

tests/tools/test_mcp_stability.py

@@ -77,7 +77,7 @@ class TestStdioPidTracking:
         from tools.mcp_tool import _stdio_pids, _lock
         with _lock:
             # Might have residual state from other tests, just check type
-            assert isinstance(_stdio_pids, set)
+            assert isinstance(_stdio_pids, dict)
 
     def test_kill_orphaned_noop_when_empty(self):
         """_kill_orphaned_mcp_children does nothing when no PIDs tracked."""
@@ -96,7 +96,7 @@ class TestStdioPidTracking:
         # Use a PID that definitely doesn't exist
         fake_pid = 999999999
         with _lock:
-            _stdio_pids.add(fake_pid)
+            _stdio_pids[fake_pid] = "test"
 
         # Should not raise (ProcessLookupError is caught)
         _kill_orphaned_mcp_children()
@@ -105,40 +105,49 @@ class TestStdioPidTracking:
             assert fake_pid not in _stdio_pids
 
     def test_kill_orphaned_uses_sigkill_when_available(self, monkeypatch):
-        """Unix-like platforms should keep using SIGKILL for orphan cleanup."""
+        """SIGTERM-first then SIGKILL after 2s for orphan cleanup."""
         from tools.mcp_tool import _kill_orphaned_mcp_children, _stdio_pids, _lock
 
         fake_pid = 424242
         with _lock:
             _stdio_pids.clear()
-            _stdio_pids.add(fake_pid)
+            _stdio_pids[fake_pid] = "test"
 
         fake_sigkill = 9
         monkeypatch.setattr(signal, "SIGKILL", fake_sigkill, raising=False)
 
-        with patch("tools.mcp_tool.os.kill") as mock_kill:
+        with patch("tools.mcp_tool.os.kill") as mock_kill, \
+             patch("time.sleep") as mock_sleep:
             _kill_orphaned_mcp_children()
 
-        mock_kill.assert_called_once_with(fake_pid, fake_sigkill)
+        # SIGTERM, then alive-check (signal 0), then SIGKILL
+        mock_kill.assert_any_call(fake_pid, signal.SIGTERM)
+        mock_kill.assert_any_call(fake_pid, 0)  # alive check
+        mock_kill.assert_any_call(fake_pid, fake_sigkill)
+        assert mock_kill.call_count == 3
+        mock_sleep.assert_called_once_with(2)
 
         with _lock:
             assert fake_pid not in _stdio_pids
 
     def test_kill_orphaned_falls_back_without_sigkill(self, monkeypatch):
-        """Windows-like signal modules without SIGKILL should fall back to SIGTERM."""
+        """Without SIGKILL, SIGTERM is used for both phases."""
         from tools.mcp_tool import _kill_orphaned_mcp_children, _stdio_pids, _lock
 
         fake_pid = 434343
         with _lock:
             _stdio_pids.clear()
-            _stdio_pids.add(fake_pid)
+            _stdio_pids[fake_pid] = "test"
 
         monkeypatch.delattr(signal, "SIGKILL", raising=False)
 
-        with patch("tools.mcp_tool.os.kill") as mock_kill:
+        with patch("tools.mcp_tool.os.kill") as mock_kill, \
+             patch("time.sleep") as mock_sleep:
             _kill_orphaned_mcp_children()
 
-        mock_kill.assert_called_once_with(fake_pid, signal.SIGTERM)
+        # SIGTERM phase, alive check raises (process gone), no escalation
+        mock_kill.assert_any_call(fake_pid, signal.SIGTERM)
+        assert mock_sleep.called
 
         with _lock:
             assert fake_pid not in _stdio_pids