Fix: Meta Description: SEO plugin detection cache never expires and skips the "no plugin" case

[hbhalodia]

What?

Closes #970

Why?

1. The positive result is stored with no expiry and is only cleared on deactivated_plugin - a hook that's registered only while the Meta Description experiment is enabled. So the cache can go stale and never self-heal.
2. The "no SEO plugin active" result is never cached, so a full plugin scan runs on every call, including the front-end output_meta_description() path.

How?

• Updates the transient to cache it for a day.
• Update the activated, deactivated hook outside of experiment, so a cache would invalidate regardless of experiment is enabled or not.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Opus 5
Used for: Code, Unit Test under the direction provided by me. Implementation reviwed by me.

Testing Instructions

1. Activate a supported SEO plugin (e.g. Yoast) + enable the Meta Description experiment; trigger meta handling once to cache the slug.
2. Disable the Meta Description experiment.
3. Deactivate the SEO plugin.
4. Re-enable the experiment, This should use the plugin's meta key, i.e wpai_meta_description.

Changelog Entry

Fixed - Cached the active SEO plugin detection - with a TTL and immediate invalidation on any plugin activation/deactivation - so meta description meta-key lookups no longer re-scan active plugins on every request.

[github-actions]

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

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[codecov]

Codecov Report

❌ Patch coverage is 60.60606% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.54%. Comparing base (8016220) to head (b482d0b).

Files with missing lines	Patch %	Lines
...des/Abilities/Meta_Description/SEO_Integration.php	55.55%	12 Missing ⚠️
includes/Admin/Upgrades/V1_4_0.php	80.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #973      +/-   ##
=============================================
- Coverage      74.57%   74.54%   -0.04%     
- Complexity      3132     3141       +9     
=============================================
  Files            132      133       +1     
  Lines          12213    12237      +24     
=============================================
+ Hits            9108     9122      +14     
- Misses          3105     3115      +10     

Flag	Coverage Δ
unit	74.54% <60.60%> (-0.04%)	⬇️

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[dkotter]

Things look good to me, thanks! Holding on merging this for now though as we add a new upgrade routine here that has the file name hardcoded to v1.4.0. If we end up doing a minor release (v1.3.1 as an example) prior to releasing 1.4.0, this code will be inaccurate. Will merge this once we know for sure the version we'll be releasing this as