Refactor to extract Abilities base class

[priethor]

What

Refactor Abilities API classes to extract a shared base class and consolidate tests.

Why

Post Type and Taxonomy abilities shared most code and logic, since they map almost 1:1 the acf_internal_post_type methods. This will reduce maintenance, simplify tests, and make it easier to add abilities for Field Groups and Options Pages.

The base class is heavily tested, with full coverage I wouldn't consider worth in most cases, but apart from leveraging ai tools to create the tests, it serves as a way to "document" the settings and allowed polishing the abilities, as some of them were not correctly marked as read-only.

How

• Extracts SCF_Internal_Post_Type_Abilities base class with shared CRUD logic
• Simplifies SCF_Post_Type_Abilities and SCF_Taxonomy_Abilities to thin subclasses that simply map to each SCF post type.
• Consolidates E2E tests into a single parameterized suite, testing both entity types
• Adds more PHPUnit tests for the base class
• Rename files to follow WordPress naming conventions:

Testing Instructions

1. Run PHPUnit tests: vendor/bin/phpunit --testsuite=secure-custom-fields
2. Run E2E tests: npm run test:e2e -- abilities-internal-post-types.spec.ts
3. More importantly, connect through MCP, list the SCF abilities and verify they work the same.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 9.36%. Comparing base (6124103) to head (21f9fa9).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff             @@
##             trunk    #264      +/-   ##
==========================================
+ Coverage     8.84%   9.36%   +0.52%     
+ Complexity    4322    4288      -34     
==========================================
  Files          265     266       +1     
  Lines        33404   33159     -245     
==========================================
+ Hits          2953    3105     +152     
+ Misses       30451   30054     -397     

Flag	Coverage Δ
e2e	21.79% <ø> (ø)
phpunit	6.08% <100.00%> (+0.63%)	⬆️

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[github-actions]

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Props priethor.

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