Add unit tests for RegisteredDataHandler

[noruzzamans]

Description of the Change

Adds a comprehensive unit test suite for the Distributor\RegisteredDataHandler class introduced in Distributor 2.2.0, which handles registered data processing across post content, Gutenberg blocks, shortcodes, post meta, and taxonomy terms.

Prior to this PR, Distributor\RegisteredDataHandler had 0% unit test coverage. This PR provides 100% test-only coverage (29 unit tests, 49 assertions) covering:

1. Instantiation & Connection State: Verifies default empty state and custom connection array assignment.
2. Block Inner Content Replacements (search_replace_block_inner_content): Tests empty replacement handling and multi-string replacements across both innerHTML and innerContent.
3. Term Extra Data Preparation (prepare_registered_data_term): Tests invalid/null/WP_Error term handling, flat taxonomy terms, and hierarchical terms with recursive parent data.
4. Term Data Processing (process_registered_data_term): Tests empty term data, existing terms, hierarchy updates when parents differ, new term insertion via wp_insert_term(), and error handling when term creation fails.
5. Post Meta Data Processing (process_registered_post_meta_data): Tests missing callbacks/keys, single string meta keys, single-element array wrapped keys, and multi-key batch updates.
6. Block Content Processing (process_blocks_data_recursive & process_registered_block_data): Tests uncallable callbacks, matching block attribute updates, media block replacement (URL, wp-image-* class, and GUID updates), nested innerBlocks recursion, and fallback when has_block() returns false.
7. Shortcode Processing (process_registered_shortcode_data): Tests fallback when has_shortcode() returns false, single attribute replacements, and multiple attribute replacements.
8. Pipeline Orchestration (process_registered_data & pre_process_registered_data_post): Tests filter bypass (dt_process_extra_data), empty registry, meta pipeline dispatching, and external connection pre-processing guards.

How to test the Change

Run the PHPUnit test suite locally:

composer test

Or run the dedicated test class directly:

vendor/bin/phpunit --filter RegisteredDataHandlerTest

All 29 tests should execute and pass without errors or regressions.

Changelog Entry

Added - Unit tests for RegisteredDataHandler.

Credits

Props @noruzzamans

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.