fix(mcp): clear backendPromise in onClose to allow backend recreation

[mlugo-apx]

Summary

Fixes #40018

When onClose fires, the handler was disposing the backend but not clearing backendPromise. This caused subsequent tool calls to await the stale promise and receive a disposed backend, resulting in "Not connected" errors.

Changes

Before:

const onClose = () => backendPromise?.then(b => backendManager.disposeBackend(b)).catch(serverDebug);

After:

const onClose = () => {
  const p = backendPromise;
  backendPromise = undefined;
  p?.then(b => backendManager.disposeBackend(b)).catch(serverDebug);
};

Testing

• Tested locally for 6+ hours with instrumentation logging
• Connection remained stable throughout
• Fix allows automatic backend recreation after any disconnect event

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