fix(cli): double-Esc cancels current prompt, not history (#784)

[Pruthvik-P]

Closes #784

What

This change makes “Esc Esc” only cancel the in-flight generation, instead of wiping out the entire chat history.

• Introduced escCount in TextBuffer to track consecutive Escape presses.
• Added an optional onCancelCurrent callback that consumers (e.g. the CLI session) can hook to abort only the current generation.
• Modified handleInput so:
  1. First Esc increments escCount and returns false (no re-render).
  2. Second consecutive Esc resets escCount, invokes onCancelCurrent, and returns true to trigger a re-render.
  3. Any other key resets escCount to zero.
• Added Jest unit tests in text-buffer.test.ts covering:
  • Single Esc does not cancel and does not re-render.
  • Double Esc calls onCancelCurrent exactly once and requests a re-render.
  • Non-Esc keys between Esc presses reset the counter.

How to test

1. Run the TypeScript tests:

   npm test  # or `yarn test`

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[Pruthvik-P]

I have read the CLA Document and I hereby sign the CLA