Editor: Skip auto-attaching media on save when attachment pages are enabled

[faisalahammad]

When attachment pages are enabled, the block editor attaching an unattached image to the post on save moves its attachment page to a URL under that post. The old attachment page URL then returns a 404, so existing links and indexed URLs break.

This adds an autoAttachMediaEnabled default block editor setting in get_default_block_editor_settings(). It is false when the wp_attachment_pages_enabled option is on, and true otherwise. Sites can still change it with the block_editor_settings_all filter.

This backports WordPress/gutenberg#84851 (issue WordPress/gutenberg#84831). The auto-attach behavior came from WordPress/gutenberg#81977.

Testing

1. Enable attachment pages: wp option update wp_attachment_pages_enabled 1.
2. Upload an image in the Media Library so it is not attached to any post, and note its attachment page URL.
3. Add the image to a post with an Image block and save the post.
4. The image stays unattached, and the old attachment page URL still loads.
5. Run wp option update wp_attachment_pages_enabled 0 and repeat. The image is now attached on save.

Unit tests: vendor/bin/phpunit tests/phpunit/tests/blocks/editor.php

Trac ticket: https://core.trac.wordpress.org/ticket/66321

Use of AI Tools

AI assistance: Yes. Tool: Claude Code. Used for drafting the patch, test, and PR text. I reviewed and tested the final code.

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[andrewserong]

This is testing well for me, and makes sense to explicitly set the default value here alongside the other block editor defaults!

Looking good to me, I can commit this later on today.

Thanks again for the fix!

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 64385
GitHub commit: 7d64ecf

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.