Tests: Add PHPUnit tests for REST API classes

Add comprehensive tests for the REST API integration:

- test-acf-rest-api.php: Tests for ACF_Rest_Api class including
  initialization, schema generation, field loading, and filtering
- test-acf-rest-api-functions.php: Tests for helper functions like
  acf_get_field_rest_schema() and acf_format_value_for_rest()
- test-acf-rest-embed-links.php: Tests for ACF_Rest_Embed_Links class
  covering link preparation and response integration
- test-acf-rest-request.php: Tests for ACF_Rest_Request class
  including request parsing and URL parameter handling

Test coverage includes:
- REST API initialization and configuration
- Schema generation and structure validation
- Field loading with show_in_rest filtering
- Request parsing and object type detection
- Embed link preparation and loading
- Filter hook integration (acf/rest/* hooks)
- Admin mode detection
- REST API format settings (light/standard)
- Error handling and edge cases

143 tests with 200 assertions. Some tests are skipped in WorDBless
environment due to field type and location type registration limitations.

Author: priethor