Media: Fix ARIA labelling and list semantics in the media views

[itzmekhokan]

The Edit attachment modal in the Media Library grid opens without an accessible name, and the attachments list is reported as structurally invalid because its items override the list-item role.

tmpl-media-modal labeled its role="dialog" element with a fixed media-frame-title ID that sat on the frame's heading container. In the grid that ID belongs to the manage frame's empty heading, and any page opening more than one media frame produces duplicates. The ID is now generated per modal instance and set on the frame's h1. Separately, the attachments ul gets role="group", since its li children carry role="checkbox" and ARIA in HTML only permits that when the list role changes too.

Trac ticket: https://core.trac.wordpress.org/ticket/66025

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: reproducing the violations in the browser, diagnosing the duplicate ID, and drafting the fix. All changes were reviewed and validated by me.

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[afercia]

Thank you @itzmekhokan the new unique ID seem to work nicely.
I'd just wait for feedback from @joedolson to confirm the role=group for the attachments list is the most appropriate one. Also to consider whether it needs to be labeled, not sure because there is already a visually hidden heading before it.

Hint for testing:

• Create a post and add a Classic block.
• When editing the Classic block, click 'Add Media' in the TinyMCE editor toolbar.
• That will give you access to a media modal dialog with all the frames like Add media, Create Gallery, Create audio playlist etc. so that all the media frames can be tested when the heading changes dynamically.