Alt Text Generation: Integrate with Decorative Image settings

[yogeshbhutkar]

What, Why, and How?

Closes #1030

This PR integrates Alt Text Generation with the Image block’s decorative setting. It disables generation for images marked as decorative and prevents applying an existing suggestion after the image is marked decorative. When AI identifies an image as decorative, it presents a suggestion with a “Mark as decorative” action, keeping the decision with the user.

The action mirrors core by clearing alt text and setting caption and link attributes to undefined. If supported, it also sets isDecorative, leaving core to render role="none" on the front-end. On editors without support, it clears the same attributes but omits the flag (isDecorative).

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-6 Astra
Used for: Multiple rounds of code-reviews to stress test the changes, and to generate the PR description.

Testing Instructions

1. Enable Alt Text Generation and configure an AI provider.
2. Add an Image block with alt text, a caption, and a link.
3. Mark it decorative and verify generation is disabled.
4. Unmark it and verify generation is enabled.
5. Trigger a decorative AI response and verify the suggestion appears.
6. Accept the suggestion and verify alt text, caption, and link are cleared.
7. Save and reload. Verify the decorative setting persists and core renders role="none".
8. On an unsupported editor, verify the same fields clear without setting isDecorative.

Screencast

alt-text-screencast.mp4

Changelog Entry

Changed - Integrates Alt Text Generation with the Image block’s decorative setting.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.72%. Comparing base (b71f930) to head (b69779d).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #1031   +/-   ##
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  Coverage      80.72%   80.72%           
  Complexity      2998     2998           
==========================================
  Files            124      124           
  Lines          11978    11978           
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  Hits            9669     9669           
  Misses          2309     2309           

Flag	Coverage Δ
unit	80.72% <ø> (ø)

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Co-authored-by: yogeshbhutkar <yogeshbhutkar@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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[dkotter]

Left a few comments to discuss.

May be nice also to add E2E tests specific to these decorative paths to ensure they work. And documentation could also be updated to mention this.