HTML API: Limit leading newline serialization to HTML elements

[sirreal]

Summary

• Restrict the serializer leading-newline preservation rule for textarea, pre, and listing to tokens in the HTML namespace.
• Add regression coverage for foreign SVG and MathML textarea elements, HTML integration-point textarea behavior, and pre / listing foreign-content exit behavior.

Root Cause

WP_HTML_Processor::serialize_token() applied the HTML textarea / pre / listing leading-LF restoration rule by tag name alone. Foreign SVG and MathML elements with local name textarea do not have that parser-stripped leading newline, so normalization inserted an extra LF and was not idempotent.

Verification

• WP_TESTS_SKIP_INSTALL=1 ./vendor/bin/phpunit --group html-api,html-api-html5lib-tests
• ./vendor/bin/phpcs src/wp-includes/html-api/class-wp-html-processor.php tests/phpunit/tests/html-api/wpHtmlProcessor-serialize.php

Review Notes

Three contrarian subagent review passes were run for the test, implementation, and final formatted diff. The requested codex-review-loop skill was invoked after commit, but codex review could not complete: the sandboxed run failed and the required unsandboxed retry was rejected by policy because it may transmit repository diff data externally.

[github-actions]

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[sirreal]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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