HTML API: Preserve decoder match length on named-reference miss

[sirreal]

Summary

• Preserve the by-reference match length when WP_HTML_Decoder::read_character_reference() misses a named reference.
• Check for null from WP_Token_Map::read_token() to match the token-map API contract.
• Add coverage ensuring unmatched named references do not mutate the caller's match length.

Testing

• Focused decoder PHPUnit coverage for unmatched named references passes.
• PHPCS alignment fixes are committed.
• 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: Splitting the fuzzer-discovered fix, 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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