Editor: Always iframe the post editor for block themes

[t-hamano]

Alternatives to #74042

What?

Make the post editor always render inside an iframe when a block theme is active.

Why?

We aim to always render the post editor's canvas as an iframe in the future. This PR extends the conditions for forcing an iframe as a preliminary step towards a full migration, in accordance with the explanation in the 7.1 roadmap.

Testing Instructions

1. Run IS_GUTENBERG_PLUGIN=false npm run dev
2. Open the post editor.
3. Register the v2 block using the following code.

wp.blocks.registerBlockType( 'my-plugin/v2-block', {
	apiVersion: 2,
	title: 'v2 block',
	edit: function () {
		return wp.element.createElement( 'p', wp.blockEditor.useBlockProps(), `v2 block` );
	},
} );

4. Insert the v2 block.
5. The editor remains an iframe.

Use of AI Tools

This PR was authored with the assistance of Claude Code (Claude Opus). All changes were reviewed by the author.

[github-actions]

Size Change: +13 B (0%)

Total Size: 7.61 MB

📦 View Changed

Filename	Size	Change
build/modules/boot/index.min.js	51.1 kB	-3 B (-0.01%)
build/scripts/edit-post/index.min.js	52.2 kB	+17 B (+0.03%)
build/scripts/edit-site/index.min.js	298 kB	-1 B (0%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in 9cfd3f7.
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/28587848589
📝 Reported issues:

• [Flaky Test] activate block theme when live previewing in edit mode #77712 in /test/e2e/specs/site-editor/activate-theme.spec.js
• [Flaky Test] shows post-locked modal when document size limit is exceeded #78753 in /test/e2e/specs/editor/collaboration/http-only/collaboration-document-size-lock.spec.ts

[t-hamano]

Closing this in favor of #74042