Editor: Add pattern support to PHP-only blocks.

[priethor]

Trac ticket: https://core.trac.wordpress.org/ticket/65628

Gutenberg PR: WordPress/gutenberg#80182

What

Extends auto-registered blocks (PHP-only blocks) so they can be edited directly in the canvas through patterns.

Instead of relying on a render_callback, block authors can provide regular block markup as a pattern. The pattern stays synced, and block authors can choose which fields can be overridden for each instance.

Why

PHP-only blocks currently rely on server-rendered previews in the editor and can only be edited through the inspector sidebar.

Testing Instructions

1. Start the local environment.
2. Run npm run test:php -- --group 65628.
3. Confirm all pattern rendering and auto-registration bootstrap tests pass.

The complete editor flow requires the JavaScript changes from Gutenberg PR #80182 and cannot be tested in Core alone until the related package update lands.

Use of AI Tools

AI assistance: Yes
Tool(s): OpenAI Codex
Model(s): GPT-5.6
Used for: Implementation support, test scaffolding, validation. I guided the work and reviewed and adjusted the resulting code and tests.

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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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