HTML API: Limit leading newline serialization to HTML elements

[sirreal]

Summary

• Limit the serializer's leading-newline preservation rule for textarea, pre, and listing to HTML namespace elements.
• Avoid inserting extra newlines for foreign SVG and MathML elements with matching local names.
• Add coverage for foreign textarea, HTML integration-point behavior, and foreign-content exits.

Testing

• HTML API and html5lib PHPUnit groups pass.
• PHPCS pass for the changed HTML API files.
• codex review --base trunk.

Trac ticket: https://core.trac.wordpress.org/ticket/65372

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5.5
Used for: PR description cleanup and code review.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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Props jonsurrell, sergeybiryukov.

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[sirreal]

Code review

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

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