test(mcp): import os and pytest in test_mcp_stability

The salvaged grandchild-reaping tests reference os.getpgid/os.killpg and
pytest.mark/skip/importorskip directly, but the file only imported asyncio,
signal, and unittest.mock. Add the missing imports so collection succeeds
on current main.

tests/tools/test_mcp_stability.py

@@ -1,9 +1,12 @@
 """Tests for MCP stability fixes — event loop handler, PID tracking, shutdown robustness."""
 
 import asyncio
+import os
 import signal
 from unittest.mock import patch, MagicMock
 
+import pytest
+
 
 
 # ---------------------------------------------------------------------------