fix: resolve Enter key not activating focused non-default dialog buttons

[maruthang]

Summary

Fixes #239062

Bug: Pressing Enter on the "Force Checkout" button (or any non-default focused button) in a dialog did not activate that button.

Root Cause: The Dialog class's Enter key handler only handled the case where an input field had focus (resolving with button index 0). When a non-default button had focus, Enter fell through to default handling and did nothing.

Fix: Added explicit Enter key handling in the Dialog class to detect when a non-default button has focus and resolve the dialog with that button's index.

Changes

• src/vs/base/browser/ui/dialog/dialog.ts: Added a check after the existing input-focus handling to iterate over the button bar's buttons and resolve with the focused button's index when Enter is pressed.
• src/vs/base/test/browser/ui/dialog/dialog.test.ts (new): Added regression tests verifying that Enter activates the correct focused button in multi-button dialogs.

Test Plan

• Added regression test in src/vs/base/test/browser/ui/dialog/dialog.test.ts that verifies Enter on a focused non-default button resolves with the correct button index
• All existing tests pass
• Manual verification: open a dialog with multiple buttons (e.g. the "Force Checkout" confirmation), tab to a non-default button, and press Enter — the focused button should now activate