Content Type: Handle duplicate slug generation for CPT's and Taxonomies

[im3dabasia]

What?

Follow up to #77600

When duplicating a post type or taxonomy, we should ensure the generated slug is not taken.

Check https://github.com/WordPress/gutenberg/pull/77938#discussion_r3188159387on auto-fill

Ensures the generated slug when duplicating a post type or taxonomy is not already taken.

Why?

buildCopySlug incremented the trailing number once without checking availability, so duplicating the same item twice pre-filled the same slug both times.

How?

buildCopySlug now loops, incrementing the trailing number, until finding a free candidate. Each modal builds a takenSlugs set from both active registered types and draft records (since duplicates are stored as drafts).

Testing Instructions

1. Enable the Content Types experiment (Gutenberg → Experiments).
2. Go to Settings → Post Types (or Taxonomies).
3. Duplicate a post type, confirm slug pre-fills as test2.
4. Duplicate it again, confirm slug is now test3, not test2.

Testing Instructions for Keyboard

Tab to a row's actions menu → Duplicate → confirm slug field is pre-filled with a free slug → submit with Enter.

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Co-authored-by: im3dabasia <im3dabasia1@git.wordpress.org>

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