Content Guidelines AI: update injection selectors for latest Gutenberg

[aagam-shah]

Proposed changes

Stacked on top of #47959.

The Gutenberg Guidelines page markup changed in ways that silently broke the Content Guidelines AI UI injection — the JS bundle loads (HTTP 200, no console error) but zero components render, because every DOM anchor the injector looks for no longer exists:

• content-guidelines__* classes were renamed to guidelines__* (Gutenberg #77223).
• The accordion was refactored to CollapsibleCard (Gutenberg #77903), which emits hashed CSS-module class names and removed the per-section element id.
• The wp-admin Page header (#77088) moved to CSS modules; it also renders no header-actions slot because the Guidelines page passes no actions to <Page>.

This PR updates _inc/content-guidelines-ai/lib/inject.js to anchor on stable selectors:

• Per-section badge / actions / generate-button key off the new data-slug attribute on .guidelines__list-item, then use semantic anchors within the section (form, the type="submit" button's HStack, the form's top VStack).
• Banner / upgrade notice use the renamed .guidelines__list.
• Header "Suggest all" button moves to the top of .guidelines__content (above the list), since there is no header-actions slot to target.
• Block-guideline modal selectors are unchanged (those classes were not renamed).

Dependency

Requires the Gutenberg-side hook: WordPress/gutenberg#78676 (data-slug on the guideline list items). Until that ships in the bundled Gutenberg, the per-section injections will no-op.

Testing instructions

1. Build the plugin and apply the Gutenberg branch from Guidelines: Add data-slug attribute to settings list items WordPress/gutenberg#78676 (or patch the attribute in).
2. Enable the gutenberg-guidelines experiment.
3. Visit wp-admin/options-general.php?page=guidelines-wp-admin&enable_ai_generation=true.
4. Confirm the AI UI now renders: the "Suggest all" button above the list, per-section badges/actions/generate buttons, and the block-guideline modal buttons.

Verification done

• Validated all 15 injection anchors resolve against the live Guidelines DOM (with the data-slug attribute simulated), and a marker-insertion test placed every component at the correct spot.
• Full in-browser render of the built bundle was not reproducible locally due to an unrelated workspace build-env issue (@automattic/jetpack-ai-client / jetpack-components build artifacts); CI/sandbox builds it normally.

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