Tests: Add PHPUnit tests for form type classes

[priethor]

What

Part of #315.

Adds PHPUnit test coverage for 8 previously untested form type classes that handle field rendering in various WordPress contexts.

Why

Form classes are critical for displaying SCF fields in different WordPress admin screens including:

• Attachment media library modal
• Comment edit screens
• Frontend forms
• Gutenberg block editor
• Nav menu item screens
• Taxonomy term add/edit screens
• User profile screens
• Widget administration

How

Adding 144 tests across all form types:

File	Class Tested	Tests
test-form-attachment.php	acf_form_attachment	14
test-form-comment.php	acf_form_comment	12
test-form-front.php	acf_form_front	19
test-form-gutenberg.php	acf_form_gutenberg	24
test-form-nav-menu.php	acf_form_nav_menu	17
test-form-taxonomy.php	acf_form_taxonomy	15
test-form-user.php	ACF_Form_User	21
test-form-widget.php	acf_form_widget	15

Test coverage includes:

• Class instantiation and constructor hook registration
• Nonce verification for save operations
• Render method behavior without matching field groups
• Admin footer JavaScript output
• Data providers for various parameter scenarios

Testing Instructions

Run the test suite:

./vendor/bin/phpunit --filter "Test_Form_Attachment|Test_Form_Comment|Test_Form_Front|Test_Form_Gutenberg|Test_Form_Nav_Menu|Test_Form_Taxonomy|Test_Form_User|Test_Form_Widget"

[github-actions]

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Core Committers: Use this line as a base for the props when committing in SVN:

Props priethor.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.13%. Comparing base (42c4ac8) to head (24f8777).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #333      +/-   ##
============================================
+ Coverage     23.67%   25.13%   +1.45%     
  Complexity     4410     4410              
============================================
  Files           291      291              
  Lines         36940    36916      -24     
============================================
+ Hits           8745     9278     +533     
+ Misses        28195    27638     -557     

Flag	Coverage Δ
e2e	28.87% <ø> (ø)
javascript	6.53% <ø> (ø)
phpunit	24.98% <ø> (+2.00%)	⬆️

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