Tests: Add more PHPUnit tests for Field classes

[priethor]

What

Part of #315, follow-up to #336.

Adds more PHPUnit tests for field type classes.

Why

Fields represent one of the biggest surfaces in SCF.

How

File	Tests	Logic Covered
test-class-acf-repeater-table.php	34	Pagination disabling, show_order/add/remove visibility, rows_per_page defaults, value padding/slicing, layout-specific rendering, collapse icons
test-class-acf-field-repeater.php	17	min/max casting, button_label defaults, non-array handling, REST schema minItems/maxItems, column_width migration
test-class-acf-field-type-group.php	14	update_value null returns, format_value_for_rest early returns, prepare_field_for_db prefixing, validate/load/delete early returns
test-class-acf-field-radio.php	10	load_value array_pop, update_value zero check, get_rest_schema enum branches
test-class-acf-field-checkbox.php	11	validate_value empty rejection, format_value acf_is_empty, get_rest_schema enum
test-class-acf-field-wysiwyg.php	9	format_value non-string return, CDATA escaping, escape_html filter
test-class-acf-field-text.php	9	validate_value maxlength boundaries, get_rest_schema maxLength casting
test-class-acf-field-link.php	12	get_link array/string/empty branches, validate_value URL checks, update_value empty handling
test-class-acf-field-oembed.php	9	prepare_field width/height defaults, format_value empty checks, get_rest_schema uri format

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

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

Props priethor.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.26%. Comparing base (af9ae43) to head (d9d2156).
⚠️ Report is 4 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #344      +/-   ##
============================================
+ Coverage     31.71%   32.26%   +0.54%     
  Complexity     4410     4410              
============================================
  Files           291      291              
  Lines         36918    36918              
============================================
+ Hits          11710    11913     +203     
+ Misses        25208    25005     -203     

Flag	Coverage Δ
e2e	42.91% <ø> (-0.51%)	⬇️
javascript	15.07% <ø> (ø)
phpunit	29.59% <ø> (+0.88%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.