Adjust alt text generation to more closely match the W3C Alt decision tree

[dkotter]

What?

Closes #350

Adjust alt text generation to better consider the W3C Alt decision tree

Why?

Currently our alt text generation experiment generates alt text that is more or less just a description of the image. While this isn't completely incorrect, it doesn't account for other situations where the alt text should be something else, like if an image is decorative or if the image is acting as a link.

Based on the feedback in #350, this PR adjusts the experiment to try and more closely match the W3C Alt decision tree, noting it's still up to the LLM to honor those guidelines.

How?

• Add a new image_meta param to our Alt Text Generation Ability
• Use this new param to pass in information about the image, like if it's wrapped in a link or the caption text
• Add support for return blank alt text and marking an image as decorative
• Update our system instructions to be more specific in their guidance

Use of AI Tools

Used both Claude Code running Opus 4.6 and Cursor running GPT-5.3 Codex to generate separate plans and execute those plans. I then compared the approaches and combined bits and pieces of both to get to a final state

Testing Instructions

1. Checkout this PR and run npm i && npm run build
2. Turn on the Alt Text Generation Experiment
3. Ensure you have a provider connected that supports vision models
4. Create a post and insert an image somewhere into the content
5. Generate alt text for that image and ensure it is accurate
6. Add a link around the image and generate it again, ensuring the alt text mentions the link
7. Generate alt text for an image in the media library, ensuring that works

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 48.00000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.77%. Comparing base (0b343a4) to head (9bcac1e).
⚠️ Report is 52 commits behind head on develop.

Files with missing lines	Patch %	Lines
includes/Abilities/Image/Alt_Text_Generation.php	45.83%	13 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #374      +/-   ##
=============================================
+ Coverage      59.75%   59.77%   +0.01%     
- Complexity       701      702       +1     
=============================================
  Files             49       49              
  Lines           3558     3572      +14     
=============================================
+ Hits            2126     2135       +9     
- Misses          1432     1437       +5     

Flag	Coverage Δ
unit	59.77% <48.00%> (+0.01%)	⬆️

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[jeffpaul]

Works well in testing, but let's hold on merging this until we determine how best to handle #340 as a potential blocker in the 0.7.0 release.