HTML API: Handle adoption agency fallback end tags

[sirreal]

Summary

• Extract the in-body "any other end tag" handling so the adoption agency algorithm can reuse it.
• Route the no-active-formatting-element adoption agency fallback through that helper instead of bailing as unsupported.
• Add serialization and semantic-rule regressions for unexpected formatting end tags and step() EOF return behavior.

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 tests/phpunit/tests/html-api/wpHtmlProcessorSemanticRules.php
• Replayed minimized fuzz artifact 1b7b7d16d7a1: first and second normalization both length 1000, SHA-1 76ef51550b1af6106e0f32d9c25422b60ab3af85.

[github-actions]

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[sirreal]

Code review

Found 1 issue:

1. The new step_in_body_any_other_end_tag() method is annotated @since 6.9.0, but trunk is at 7.1-alpha (src/wp-includes/version.php#L19) and Trac #65372 is milestoned 7.1. This should be @since 7.1.0.

wordpress-develop/src/wp-includes/html-api/class-wp-html-processor.php

Lines 3258 to 3260 in 7e4ecc7

	*
	* @since 6.9.0
	* @ignore

Also verified (no action needed): the adoption agency fallback to "any other end tag" matches the spec ("If there is no such element, then return and instead act as described in the 'any other end tag' entry above"), and the start-tag call sites (+A, +NOBR) cannot reach the fallback — +A is guarded by the active formatting elements walk, and the +NOBR paths that could diverge bail earlier.

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