fix: showMonthYearDropdown not keyboard/tab accessible

[balajis-qb]

Description

Linked issue: #6311

Problem

When showMonthYearDropdown is enabled:

• The read-view toggle (MonthYearDropdown.renderReadView) rendered as a plain <div onClick> instead of a <button type="button"> like MonthDropdown/YearDropdown, so it was skipped by Tab and didn't respond to Enter/Space.
• Once opened, the options list (MonthYearDropdownOptions) rendered each option as a plain <div> with no role, tabIndex, onKeyDown, or focus management, unlike MonthDropdownOptions/YearDropdownOptions, so the list itself couldn't be navigated by keyboard even after the toggle became reachable.

Changes

• src/month_year_dropdown.tsx: changed the read-view toggle from <div> to <button type="button">.
• src/month_year_dropdown_options.tsx: added role="button", tabIndex, and onKeyDown handling (Enter selects, Escape cancels, ArrowUp/ArrowDown moves focus between options without wrapping) plus auto-focus of the selected option when the list opens — matching the existing MonthDropdownOptions/YearDropdownOptions pattern.
• src/test/month_year_dropdown_test.test.tsx: added regression tests covering the button read view and the new keyboard navigation/focus behavior.

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.32%. Comparing base (548a1f3) to head (13313ed).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6312      +/-   ##
==========================================
+ Coverage   99.29%   99.32%   +0.03%     
==========================================
  Files          30       30              
  Lines        3822     3845      +23     
  Branches     1648     1674      +26     
==========================================
+ Hits         3795     3819      +24     
+ Misses         26       25       -1     
  Partials        1        1              

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