SEO: add an AI tab and move the AI SEO Enhancer setting

[angelablake]

Stacked on #49351 (SEO Content tab) so the test artifact shows the whole product. The diff here is AI-only. When #49351 merges, this rebases onto trunk.

Proposed changes

Adds an AI tab to the Jetpack SEO dashboard and moves the existing AI SEO Enhancer setting onto it.

This re-scopes what was originally planned as the larger "AI tab" PR: it stands up the tab shell and ports the one setting that already exists, so the bigger AI features (llms.txt, AI-crawler controls) can be built on top of it separately by their own efforts.

Why now

1. The AI SEO Enhancer toggle (ai_seo_enhancer_enabled — "Automatically generate SEO title, SEO description, and image alt text for new posts") currently lives only on the legacy SEO Traffic page. It needs a home on the new admin before that page is eventually removed.
2. Creating the tab + route gives follow-up AI work (llms.txt generation, AI-crawler management) a place to land its settings.

What this PR does

• New "AI" tab alongside Overview and Settings (_inc/app.tsx). The tab itself is not plan-gated — it stays visible on all hosting, since free AI settings will be added to it later.
• Initializer::get_ai_data() bootstraps a seo.ai slice onto the page (window.JetpackScriptData.seo.ai) with the enhancer's enabled state and whether it's available. Availability mirrors the legacy gate: the ai_seo_enhancer_enabled feature filter must be on (the feature still depends on AI being available) and the site's plan must support the ai-seo-enhancer feature (Current_Plan::supports(), guarded by class_exists as it's provided by the host plugin).
• AI screen (_inc/screens/ai/) renders the enhancer toggle only when available (parity with the legacy page); otherwise it shows a neutral placeholder so the tab isn't blank. The toggle saves on change through the existing /jetpack/v4/settings endpoint (ai_seo_enhancer_enabled) — no new REST — with the same "Updating settings…"→"Settings saved." snackbar as the Settings tab, reverting on error. State is lifted to the page root so it survives tab switches.

This is additive: the legacy Traffic-page toggle is left in place and removed later when the old page is retired. Generation itself is entirely editor/AI-Assistant side, so there is no PHP generation logic to move — only the persisted toggle and its plan-gate.

Other information

• Have you written new tests for your changes? — Initializer::get_ai_data() contract test (shape + unavailable without a supporting plan) in the SEO package suite.
• Have you added a changelog entry?

Does this pull request change what data or activity we track or use?

No. It relocates an existing setting's UI; no new options or telemetry.

Testing instructions

1. Apply this branch (Jetpack Beta) and open the Jetpack SEO admin page. Confirm an AI tab appears next to Overview and Settings.
2. On a site whose plan supports the AI SEO Enhancer, confirm the AI SEO Enhancer toggle renders on the AI tab, reflects the current value, and that toggling it persists (reload — the new value sticks) with the save snackbar.
3. Confirm the value stays in sync with the legacy Traffic-page toggle (changing one is reflected by the other), since both write the same ai_seo_enhancer_enabled option.
4. On a site without a supporting plan, confirm the AI tab is still visible and shows the placeholder (no toggle), matching the legacy page hiding the setting.

The SEO dashboard is behind the rsm_jetpack_seo feature flag; enable it on the test site to reach the page.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the add/jetpack-seo-ai-tab branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack add/jetpack-seo-ai-tab

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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[jp-launch-control]

Code Coverage Summary

No summary data is available for parent commit 9fa7f1d, so cannot calculate coverage changes. 😴

If that commit is a feature branch rather than a trunk commit, this is expected. Otherwise, this should be updated once coverage for 9fa7f1d is available.

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