Fix checkbox returning empty string on Alt-Enter

[uttam12331]

Summary

Closes #324.

In a checkbox prompt, submitting with Alt-Enter (Meta+Enter) returns the empty string "" instead of the list of selected values. Only plain Enter (Keys.ControlM) was bound to the submission handler, so the escape, c-m sequence fell through to the default and returned "".

Reproduction (before the fix):

# space to select first choice, then Alt-Enter to submit
KeyInputs.SPACE + KeyInputs.ESCAPE + KeyInputs.ENTER  ->  ''   # expected ['foo']

Fix

Bind the Alt-Enter sequence (Keys.Escape, Keys.ControlM) to the same set_answer handler as plain Enter, so it submits and returns the selected values:

@bindings.add(Keys.ControlM, eager=True)
@bindings.add(Keys.Escape, Keys.ControlM, eager=True)
def set_answer(event):
    ...

Tests

Added test_select_first_choice_with_alt_enter asserting Alt-Enter returns ['foo'] (matching the plain-Enter behavior).

Verified locally: pytest tests/prompts/test_checkbox.py → 34 passed; ruff check clean.