Post Types: Add 'at_a_glance' property and update dashboard widget logic. Fixes #45035.

[SainathPoojary]

Refreshes #10647, resolving merge conflicts with trunk and addressing outstanding review feedback.

Note on "Published" label: Trunk changed the built-in labels to "Published post / Published page" 61555. This PR uses $post_type_object->labels->name for CPTs, which does not include "Published".

Testing Instructions:

1. Install the test plugin from this gist:
   It registers three CPTs: book (dashicon, at_a_glance=true), movie (SVG icon, at_a_glance=true), and secret_type (at_a_glance=false).
2. Publish some posts of each type.
3. Check Dashboard → At a Glance:
   • Books and Movies counts should appear with their icons
   • Secret Types should NOT appear
4. Verify built-in Posts and Pages still work correctly.

Trac ticket: https://core.trac.wordpress.org/ticket/45035

Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot
Model(s): Gemini, Claude
Used for: Checking for potential edge cases, drafting the PR description, and assisting with local code review. The final implementation and testing were written and executed manually by me.

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