HTML API: Ensure correct normalization of namespace-prefixed attribute names

[sirreal]

Fixes an issue where HTML Processor serialization issue where adjusted foreign attributes such as xlink:href were emitted with a space separator as xlink href. After normalization, this would become two attributes, xlink and href.

This keeps duplicate detection keyed to the adjusted qualified attribute name while converting adjusted foreign attributes back to colon-prefixed names when writing HTML. It also adds regression coverage for both WP_HTML_Processor::normalize() and serialize_token().

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

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5
Used for: Discover issue with fuzz testing, add the regression test, implement fix, perform verification, and draft this PR description.

[github-actions]

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[sirreal]

Code review

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

Verified that the space-to-colon replacement in serialize_token() is safe: the tokenizer never permits spaces in attribute names, so a space can only appear in a qualified name via the adjusted foreign attributes table in get_qualified_attribute_name(). Keeping duplicate detection keyed on the space-form qualified name is equivalent to keying on the serialized name, since attribute names from the tag processor are unique lowercase names and the mapping is collision-free.

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