Stop ql-mcp server when the VS Code extension is uninstalled

The `ql-mcp` server needs to be stopped when the `vscode-codeql-development-mcp-server` extension is uninstalled in VS Code. The current implementation leaves the `ql-mcp` server running after extension uninstall, which remains unresolved until the next time the user restarts extensions in VS Code (or restarts the entire developer window).