Add descriptive alt text to Hyrax thumbnail render paths and overrides

[Janell-Huyck]

Summary

This PR improves thumbnail accessibility by ensuring thumbnail images have meaningful alt text across Hyrax/Blacklight render paths, including fallback and featured collection thumbnails.

Why

Some thumbnail images were rendered with weak/missing descriptions (or marked as presentational), which reduced the usefulness of those images for screen reader users.

What Changed

• Added a shared alt-text helper in ApplicationHelper:
  • thumbnail_alt_text_for(record)
  • Builds alt text from record type + label when available.
  • Falls back safely when data is incomplete.
  • Treats null-like labels (for example "null") as missing to avoid output like Collection thumbnail: null.
• Ensured default Blacklight thumbnail rendering gets alt text when none is explicitly provided via CatalogHelper#render_thumbnail_tag.
• Added/overrode Hyrax/Blacklight thumbnail partials to pass descriptive alt text where thumbnails are rendered with image_tag.
• Updated featured collections homepage rendering to use helper-generated alt text and removed presentational treatment.
• Updated featured collection feature spec to assert helper-derived alt text (resilient to text wording changes).

Accessibility Impact

• Thumbnail images now expose meaningful alt text in key UI surfaces.
• Prevents null placeholder labels from leaking into assistive text.
• Removes presentational treatment where thumbnails carry content meaning.

Risk / Notes

• This PR includes local Hyrax/Blacklight view overrides; future gem upgrades should re-validate override compatibility.
• No DB/schema changes.
• No API contract changes.

Rollback

Revert this PR to restore prior thumbnail alt text behavior.

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Affects thumbnails on the index pages only:

Uses the alt text "Collection Thumbnail: {name of collection}" or "Work Thumbnail: {name of work}" on index pages only.

Does include index page of works inside a collection:

Does not affect images inside individual collections:

[haitzlm]

This looks like an excellent update, replacing missing or unhelpful alt text with meaningful, which increases compatibility with new accessibility guidelines