Tests: Add comprehensive tests for WP_HTML_Decoder.

[huzaifaalmesbah]

Ticket: https://core.trac.wordpress.org/ticket/64225

Adds unit tests to cover named entities, numeric entities, and edge cases in WP_HTML_Decoder, achieving 100% code coverage.

Changes:

• Added test_decode_attribute_decodes_named_entities for standard named entities.
• Added test_decode_attribute_decodes_hex_numeric_entities and test_decode_attribute_decodes_decimal_numeric_entities.
• Added test_decodes_windows_1252_mapped_characters to verify C1 control mappings.
• Added edge case tests for invalid numeric entities, null bytes, and 4-byte UTF-8 characters (emojis).
• Added test_decodes_ambiguous_ampersands for context-specific behavior.

Testing:
Run npm run test:php -- --group html-api to verify. All tests pass.

[github-actions]

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[github-actions]

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