Block JSON schema & PHP-only blocks: Add autoRegister support

[priethor]

What

Part of #79330.

Adds autoRegister to the block.json schema. Block authors will now get autocomplete for the setting, and values such as "yes" will show a schema error instead of being silently accepted.

Why

autoRegister already works for PHP-only blocks, but the schema does not know about it yet.

Testing Instructions

1. Open test/integration/fixtures/block-schemas/invalid-auto-register.json in an editor with JSON schema validation.
2. Check that "autoRegister": "yes" is marked as invalid.
3. Change the value to true, check that the error disappears, then undo the change.

---

I used Claude to help implement these changes. I guided the work, then tested and reviewed the result.

[github-actions]

Size Change: 0 B

Total Size: 7.71 MB

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[github-actions]

Flaky tests detected in e088c51.
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/29242263162
📝 Reported issues:

• [Flaky Test] adds and activates a new tab when pressing Enter at the end of a tab label #80106 in /test/e2e/specs/editor/blocks/tabs.spec.js
• [Flaky Test] persists table layout across all tabs with unified view persistence #80156 in /test/e2e/specs/site-editor/pages-view-persistence.spec.js

[github-actions]

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Co-authored-by: priethor <priethor@git.wordpress.org>
Co-authored-by: jorgefilipecosta <jorgefilipecosta@git.wordpress.org>

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[jorgefilipecosta]

LGTM 👍