Fix global AI enabled option migration

[ekamran]

What?

Fixes migration of the global AI enabled setting from older AI Experiments installs.

Why?

Older versions saved the global toggle as ai_experiments_enabled.

The existing upgrade routine intended to migrate old experiment settings, but the global option used the wrong names. As a result, after upgrading from AI Experiments 0.5.0 to the current AI plugin, the global Enable AI setting could appear disabled even though it had been enabled before the upgrade.

How?

• Migrates ai_experiments_enabled to the current wpai_features_enabled option.
• Keeps support for the singular typo option names created by earlier upgrade logic.
• Repairs the global enabled option before version checks, so already-upgraded sites can recover too.
• Avoids overwriting wpai_features_enabled if the current option already exists.
• Updates upgrade tests for both fresh legacy upgrades and already-upgraded repair cases.

Testing Instructions

Automated checks run locally:

npm run test:php
npm run lint:php
npm run lint:php:stan

Manual verification:

1. Install AI Experiments 0.5.0.
2. Go to Settings > AI Experiments.
3. Enable experiments.
4. Upgrade to this branch.
5. Go to Settings > AI.
6. Confirm Enable AI remains enabled.

Screenshots/Screencast

Before: AI Experiments 0.5.0 has experiments enabled.

After: Current AI settings preserve the global enabled state after upgrade.

Use of AI Tools

AI assistance: Yes
Tool(s): ChatGPT / Codex
Used for: Repository review, reproduction planning, implementation guidance, test updates, and local verification. I reviewed the changes, tested the behavior locally, and take responsibility for the final submission.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: ekamran <ekamran@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.20%. Comparing base (841b3da) to head (169dea7).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #586      +/-   ##
=============================================
+ Coverage      72.24%   73.20%   +0.96%     
- Complexity      1162     1529     +367     
=============================================
  Files             68       77       +9     
  Lines           5609     6878    +1269     
=============================================
+ Hits            4052     5035     +983     
- Misses          1557     1843     +286     

Flag	Coverage Δ
unit	73.20% <100.00%> (+0.96%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.