[feature] Cancel SelectionPrompt on Esc (Fixes #851)

[nimobit]

Fixes #851

Changes

• Added logic in SelectionPrompt to cancel when Escape key is pressed
• Returns null to match expected behavior
• Added unit test to simulate Escape key press and assert OperationCanceledException

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Checklist

• I have read the Contribution Guidelines
• I have commented on the issue and discussed the intended changes
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• No documentation changes required

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This is my first pull request to this project.
I'm open to any suggestions or feedback to improve it!

[nimobit]

@microsoft-github-policy-service agree