The StdioClientTransport.close() method doesn't properly terminate spawned child processes, causing
memory leaks in applications that frequently connect/disconnect from STDIO MCP servers.

In client/stdio.js, the close() method only aborts the spawn signal and nullifies the process
reference, but doesn't actually kill the running child process:

async close() {
    this._abortController.abort();  // Only prevents new spawns
    this._process = undefined;      // Just removes reference
    this._readBuffer.clear();
}

To Reproduce

  FOR 1000 times:
      transport = new StdioClientTransport(server_command)
      client = new Client()

      client.connect(transport)  // spawns child process
      pid = transport.pid

      client.close()             // BUG: doesn't kill child process

      // Process with 'pid' is still running (memory leak)

  // Result: 1000 orphaned processes still running

Expected behavior
When close() is called, the spawned child process should be terminated to prevent resource leaks.