Update the "alt text loading state" button to indicate when Alter generation is in progress.

[Rajat1192]

What?
Closes #384

Updates the “Generate Alt Text" button to say Generating... when Alt text generation is happening.

Why?
While we have a loading state on these buttons, updating the text makes it more clear and matches how we do things in other places.

How?
Updates the “Generate Alt Text button to show Generating... while excerpt generation is in progress.

Use of AI Tools
N/A

Testing Instructions

1. Check out this PR branch and run npm i && npm run build.
2. Add a valid AI connector.
3. Turn on alt text generation.
4. Create a post.
5. Find the Add an Generating... button in the post status sidebar and click it to open the popover.
6. Click the Generate alt text button in the popover.
7. Ensure the text updates to Generating....
8. Once the alt text is generated, make sure the button updates to Re-generate alt text.
9. Click it again and ensure the button text updates to Generating....

### Testing Instructions for Keyboard
### Screenshots or screencast

Screencast.from.2026-04-09.12-17-11.mp4

[github-actions]

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Unlinked contributors: yusufhay.

Co-authored-by: Rajat1192 <rajat1192@git.wordpress.org>
Co-authored-by: vishwaAghera-WPdev <vishwa99@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 (4f5732e) to head (f437b1a).
⚠️ Report is 10 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #396   +/-   ##
==========================================
  Coverage      68.36%   68.36%           
  Complexity       813      813           
==========================================
  Files             53       53           
  Lines           3945     3945           
==========================================
  Hits            2697     2697           
  Misses          1248     1248           

Flag	Coverage Δ
unit	68.36% <ø> (ø)

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[jeffpaul]

Button updates as expected, but seeing oddity where all images tested come back as decorative so no Alt Text gets generated (but assuming that's an issue with my provider for now).