Tests: Add PHPUnit tests for post type and taxonomy functions

[priethor]

What

Part of #314.

Adds PHPUnit test coverage for SCF post type and taxonomy functions in acf-post-type-functions.php and acf-taxonomy-functions.php.

Why

Post type and taxonomy functions are essential for managing custom post types and taxonomies created through SCF. They handle registration, CRUD operations, import/export, and status management.

How

Adding 87 tests covering:

• Post type/taxonomy retrieval by ID and key
• Raw data retrieval
• Key validation (acf_is_post_type_key, acf_is_taxonomy_key)
• Validation and translation functions
• Collection retrieval with filtering
• CRUD operations (create, read, update, delete)
• Trash/untrash operations
• Duplication with key remapping
• Active status management
• Edit link generation
• Export/import operations
• PHP code export for registration
• Enter title placeholder export

Testing Instructions

Run the test suite:

./vendor/bin/phpunit --filter "Test_ACF_Post_Type_Functions|Test_ACF_Taxonomy_Functions"

[github-actions]

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Props priethor.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 15.91%. Comparing base (675560c) to head (f8af317).
⚠️ Report is 7 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #323      +/-   ##
============================================
+ Coverage     15.76%   15.91%   +0.15%     
  Complexity     4410     4410              
============================================
  Files           291      291              
  Lines         36997    36997              
============================================
+ Hits           5832     5888      +56     
+ Misses        31165    31109      -56     

Flag	Coverage Δ
e2e	28.87% <ø> (ø)
javascript	2.50% <ø> (ø)
phpunit	13.82% <ø> (+0.20%)	⬆️

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