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]

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Co-authored-by: Mamaduka <[email protected]>
Co-authored-by: fabiankaegy <[email protected]>

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[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! :)