Replace global Enable AI toggle with tri-state section master toggles

[itzmekhokan]

Fixes #600 by implementing Option B from the issue: remove the global Enable AI header toggle and add a tri-state section master toggle to each experiment group.

• Off — no features enabled in the section
• Indeterminate — some but not all features enabled (distinct visual treatment)
• On — all features in the section enabled
  The header toggle was misleading because it reflected only wpai_features_enabled, not the aggregate state of sub-features. Section-level controls scale better as more experiment groups are added and align with existing Enable all / Disable all actions.
  Changes also:
• Sync wpai_features_enabled automatically when feature toggles change (enabled when any feature is on, disabled when none are)
• Keep individual feature toggles interactive regardless of global option state
• Remove bulk-action disabling tied to the old global header toggle

Test plan

• Open AI settings and confirm there is no global Enable AI toggle in the page header
• Confirm each section (Editor Experiments, Admin Experiments, etc.) shows an Enable {Section} master toggle
• Enable only a subset of features in Editor Experiments → section master toggle shows indeterminate (not fully on)
• Click indeterminate section toggle → all features in that section enable
• Click fully-on section toggle → all features in that section disable
• Enable all / Disable all still work and remain enabled in mixed states
• Individual feature toggles remain interactive when a section is fully disabled
• Run npm run test:e2e -- tests/e2e/specs/admin/settings.spec.js
• Run npm run test:php

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.18%. Comparing base (3624c91) to head (3c2a8a6).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #617   +/-   ##
==========================================
  Coverage      73.18%   73.18%           
  Complexity      1731     1731           
==========================================
  Files             85       85           
  Lines           7473     7473           
==========================================
  Hits            5469     5469           
  Misses          2004     2004           

Flag	Coverage Δ
unit	73.18% <ø> (ø)

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[jeffpaul]

The new toggle added feels duplicative with the Enable all // Disable all buttons at the end of each section:

The buttons feel sufficient, though perhaps an alternate UI treatment instead of two distinct buttons could be better so I'm open to ideas/discussion there.