feat(gateway): add MCP server shutdown on gateway exit

Ensures MCP subprocess connections are closed when the messaging
gateway shuts down, preventing orphan processes.

gateway/run.py

@@ -2203,6 +2203,13 @@ async def start_gateway(config: Optional[GatewayConfig] = None) -> bool:
     cron_stop.set()
     cron_thread.join(timeout=5)
 
+    # Close MCP server connections
+    try:
+        from tools.mcp_tool import shutdown_mcp_servers
+        shutdown_mcp_servers()
+    except Exception:
+        pass
+
     return True