fix: Generate Button UX for title generation

[Takshil-Kunadia]

What?

Closes #385

Updates the Title Generation standalone button's loading state to match the standard loading pattern used elsewhere in the plugin (e.g. Excerpt Generation) adding the "Generating…" label and disabling the button while the request is in flight.

Why?

Match the Button UI with other functionalities.

How?

• Three labels: Generate > Generating… > Re-generate.
• Added disabled state when request is inflight.

Use of AI Tools

Testing Instructions

1. In WP Admin, enable the Title Generation experiment.
2. Normal editing mode:
   1. Create a new post with some body content and an empty title.
   2. Click into the title field and the standalone button should appear above it reading Generate.
   3. Click it. While the request is in flight, verify:
      • The label reads Generating…
      • The button is disabled and cannot be re-clicked
      • The standard busy indicator renders on the button
   4. Once the title populates, verify the button returns to Re-generate and is enabled again.
3. Template editing mode:
   1. Open a template that contains the Post Title block.
   2. Select the Post Title block → the Re-generate button appears in the block toolbar.
   3. Click it and verify the same three-state label behavior (Generate / Generating… / Re-generate) and that the button is disabled during the request.
4. Tip: throttle the network to "Slow 3G" in DevTools if the request is too fast to observe the loading state.

Screenshots or screencast

Before	After

Changelog Entry

Fixed - Update the Title Generation button's loading state to match the standard loading pattern used across other AI experiments.

[github-actions]

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Co-authored-by: Takshil-Kunadia <takshil@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.36%. Comparing base (0131263) to head (6220cf7).
⚠️ Report is 8 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #433   +/-   ##
==========================================
  Coverage      68.36%   68.36%           
  Complexity       813      813           
==========================================
  Files             53       53           
  Lines           3945     3945           
==========================================
  Hits            2697     2697           
  Misses          1248     1248           

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[jeffpaul]

Works well in my testing, thanks!