Plugin: Fix eligibility check for post types' default rendering mode

[Mamaduka]

What?

Fixes #67875.

PR fixes checks for adding a new default_rendering_mode to the post-type object, ensuring that the current theme supports block-template.

Testing Instructions

1. Disabled block-template support for block theme.
2. Open a page.
3. Confirm that its content is correctly rendered.

Snippet

add_action( 'after_setup_theme', function() {
	remove_theme_support( 'block-templates' );
} );

Testing Instructions for Keyboard

Same.

[Mamaduka]

This is a rare case, but technically doable.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Unlinked Accounts

The following contributors have not linked their GitHub and WordPress.org accounts: @CreativeDive.

Contributors, please read how to link your accounts to ensure your work is properly credited in WordPress releases.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Unlinked contributors: CreativeDive.

Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: fabiankaegy <fabiankaegy@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Flaky tests detected in cbeea5f.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/12298386191
📝 Reported issues:

• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js

[Mamaduka]

If there are no objections, I'm going to merge this :)

[fabiankaegy]

Good catch! :)