Administration: Support custom post statuses in the Publish meta box

[Sukhendu2002]

Adds explicit custom post status support to the classic editor’s Publish meta box. Primary Publish behavior remains unchanged. Broader status capabilities, post-type registration, REST API, and block editor behavior remain outside this PR.

Trac ticket: https://core.trac.wordpress.org/ticket/12706

Testing

• npm run test:php -- --filter Tests_Admin_MetaBoxes
  • 2 tests, 7 assertions passed.
• npx grunt qunit
  • Compiled and uncompiled suites passed.
• JSHint passed for post.js and QUnit tests.
• Targeted PHPCS passed.
• git diff --check passed.

Manual testing

1. Register a custom post status.
2. Add its slug through post_submitbox_custom_statuses for posts.
3. Open a post in the classic editor.
4. Confirm the status appears in the Publish meta box.
5. Select it and confirm the button displays “Save as [status]”.
6. Toggle visibility from Public → Private → Public.
7. Confirm the custom status and save-button text are restored.

Use of AI Tools

AI assistance: Yes
Tool(s): Pi coding agent
Model(s): GPT-5.6
Used for: Ticket analysis, initial code skeleton, and code review; final implementation and tests were reviewed and edited by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

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