Tests: Expand JavaScript unit tests for field bindings

[priethor]

What

Part of #315.

Expands JavaScript unit test coverage for the block bindings module (assets/src/js/bindings/).

Why

Block bindings enable connecting SCF fields to WordPress block attributes. These utilities handle field processing, metadata caching, editor context detection, and binding source registration. Tests ensure the binding system works correctly across post and site editors.

How

Adding 70 tests covering:

New test files:

• block-editor.test.js - HOC filter registration and module setup
• constants.test.js - BLOCK_BINDINGS_CONFIG structure and field type mappings

Expanded test files:

• field-processing.test.js - All field types with parameterized tests, full branch coverage
• hooks.test.js - Cleanup/cancellation, null handling, template edge cases
• sources.test.js - canUserEditValue, registration validation, edge cases

Note: block-editor.js has low coverage because the HOC requires a full React rendering environment with WordPress data stores.

Testing Instructions

Run the test suite: npm run test:unit -- --testPathPattern="tests/js/bindings"

[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 21.20%. Comparing base (402b2cc) to head (63020ae).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #330      +/-   ##
============================================
+ Coverage     21.19%   21.20%   +0.01%     
  Complexity     4410     4410              
============================================
  Files           291      291              
  Lines         36940    36940              
============================================
+ Hits           7828     7832       +4     
+ Misses        29112    29108       -4     

Flag	Coverage Δ
e2e	28.87% <ø> (ø)
javascript	2.57% <ø> (+0.06%)	⬆️
phpunit	21.06% <ø> (ø)

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