Improve a11y of Meta Description modal controls

[t-hamano]

Part of #180

What?

This PR fixes some issues related to a11y in the meta description modal.

Why? How?

The key points of the problem and solution are as follows:

• Depending on the specific state, the disabled attribute is added to the button. Buttons with the disabled attribute are not focusable, leading to a loss of focus. To resolve this, the accessibleWhenDisabled prop is available.
• When the description is copied, a snackbar appears, but it does not notify the screen reader. In the first place, the snackbar itself is not appropriate because the modal background is expected to be non-interactive. This PR the button text when copied and notifies the screen reader via the speak() method that the copy action has been performed.

Here are some additional changes I'd like to propose, but they are outside the scope of this PR.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 4.7
Used for: Refactoring the copy button to match Gutenberg's CopyButton pattern and wiring up accessibleWhenDisabled. All output was reviewed by me.

Testing Instructions

Generate Meta Description

1. Open the Meta Description Modal.
2. Wait for the automatic generation of the meta description to complete.
3. Click the Generate button.
   • Before: The focus is lost.
   • After: The focus remains on the Generate button.

generate-buton.mp4

Close Meta Description Modal

1. Open the Meta Description Modal.
2. Close the modal before the description generation is complete.
   • Before: The focus is lost.
   • After: Focus returns to the button that triggered the modal.

Incidentally, since the "Generate Meta Description" button is a button to trigger a modal, the isBusy and disabled props should not be necessary in the first place. Ideally, the description generation should be canceled when the modal is closed, but this is out of scope for this PR.

close-modal.mp4

Copy Meta Description

1. Open the Meta Description Modal.
2. Wait for the automatic generation of the meta description to complete.
3. Click the "Copy to clipboard" button.
   • Nothing is announced by the screen reader.
   • The screen reader announces Meta description copied to clipboard..

copy.mp4

Changelog Entry

Changed - Improve accessibility of the Meta Description modal: inline "Copied!" confirmation on the copy button and accessibleWhenDisabled on disabled controls.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.41%. Comparing base (e63d8c0) to head (37b5e87).
⚠️ Report is 21 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #445   +/-   ##
==========================================
  Coverage      67.41%   67.41%           
  Complexity       907      907           
==========================================
  Files             59       59           
  Lines           4352     4352           
==========================================
  Hits            2934     2934           
  Misses          1418     1418           

Flag	Coverage Δ
unit	67.41% <ø> (ø)

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[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[t-hamano]

This implementation is almost identical to this one.