fix(AlertDialog): fire onCancel when dialog is dismissed via Escape key

[EduardF1]

Summary

Fixes #1773

When a user presses Escape to close an AlertDialog, the onCancel callback was not being invoked. This happened because the Escape key is intercepted by the Modal/Tray overlay (which calls state.close() directly) and completely bypasses the cancel button's onPress handler that chains onClose() + onCancel().

Root Cause

In AlertDialog.tsx, the cancel button wires up:
sx onPress={() => chain(onClose(), onCancel())}

But when Escape is pressed, the Modal overlay handles it by calling state.close() directly, never touching onCancel. The AlertDialog had no mechanism to intercept this.

Fix

Provide a custom DialogContext inside AlertDialog that injects an onKeyDown handler. When the dialog's section element receives a keydown event for Escape and onCancel was provided, the callback fires before the overlay closes the dialog.

The onKeyDown reaches the section element via the existing useDialog(mergeProps(contextProps, props)) call in Dialog.tsx, which passes all valid DOM attributes (including onKeyDown) through filterDOMProps into the rendered element.

Tests

Added two new test cases to AlertDialog.test.js:

• Escape fires onCancel when the prop is provided
• No unexpected calls when onCancel is not provided

[LFDanLu]

@EduardF1 Thanks for the PR, the approach looks good to me. Mind signing the CLA so we can move forward with this? As an aside, we should probably implement this for the Spectrum 2 Alert Dialog as well, but no need to handle that here