Testing: Add PHPUnit tests for field group functions

[priethor]

What

Part of #315.

Adds PHPUnit test coverage for SCF field group functions.

Why

Field groups are the core organizational unit in SCF. These functions handle CRUD operations, visibility rules, duplication, and import/export.

How

With the help of Claude, adding 66 tests covering:

• Field group key validation
• Field group validation and defaults
• CRUD operations (get, update, delete, trash, untrash)
• Collection operations and filtering
• Visibility and location rule matching
• Field group duplication
• Active status management
• Style generation for hide_on_screen
• Import/export functionality
• Settings tabs configuration
• Cache and slug filter functions

Testing Instructions

Run the test suite: ./vendor/bin/phpunit --filter Test_ACF_Field_Group_Functions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 14.17%. Comparing base (ab02e04) to head (48eeeeb).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #317      +/-   ##
============================================
+ Coverage     13.55%   14.17%   +0.61%     
  Complexity     4404     4404              
============================================
  Files           291      291              
  Lines         36984    36984              
============================================
+ Hits           5014     5241     +227     
+ Misses        31970    31743     -227     

Flag	Coverage Δ
e2e	28.87% <ø> (ø)
javascript	2.50% <ø> (ø)
phpunit	11.43% <ø> (+0.84%)	⬆️

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