Fix Alt-Text generation on decorative images resulting in false error message in Media Lib

[Intenzi]

What?

Closes #557

This PR resolves the false image alt-text generation error observed when an image is decorative.

Why?

It improves the UX flow and ensures the ambiguous error is fixed.

How?

• Utilises generateAltText() method in media.ts and bulk.ts of alt-text-generation experiment.
• Adds 'Image identified as decorative. Alt text cleared.' text in media library to enhance UX and properly resolve the earlier ambiguous error.

Use of AI Tools

Testing Instructions

1. Configure an AI Connector and enable the Alt Text Generation experiment.
2. Upload an image the model is likely to mark decorative (e.g. a thin divider line, a solid color block, a purely ornamental flourish).
3. Single image: open the attachment in the Media Library, click Generate in the alt text panel.

• Observe: "Image identified as decorative. Alt text cleared" success result.

Screenshots or screencast

Changelog Entry

Fixed - False error for Alt-Text generation on decorative images in media library

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.21%. Comparing base (7888332) to head (6fcb228).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #559   +/-   ##
==========================================
  Coverage      71.21%   71.21%           
  Complexity      1151     1151           
==========================================
  Files             67       67           
  Lines           5565     5565           
==========================================
  Hits            3963     3963           
  Misses          1602     1602           

Flag	Coverage Δ
unit	71.21% <ø> (ø)

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[github-actions]

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