HTML API: Handle "adoption agency" any other end tag case

[sirreal]

Implement adoption agency algorithm's "any other end tag" case.

This does not fully implement the remaining adoption agency algorithm support, only the "any other end tag" case. Additional support can be added separately.

Notably this supports HTML like the following, which the WordPress editor is known to produce:

<a><a></a></a>

(Correctly produces to A siblings on this branch, unsupported in 7.0)

Compared with trunk, 6 html5lib tests are now supported:

• <b><table><td></b><i></table>X
• <select><b><option><select><option></b></select>X (this is legacy SELECT behavior, see HTML API: Update customizable <select> parsing #9298)
• <b>Test</i>Test
• <b><table><td></b><i></table>
• <select><b><option><select><option></b></select> (this is legacy SELECT behavior, see HTML API: Update customizable <select> parsing #9298)
• </strong></b></em></i></u></strike></s></blink></tt></pre></big></small></font></select></h1></h2></h3></h4></h5></h6></body></br></a></img></title></span></style></script></table></th></td></tr></frame></area></link></param></hr></input></col></base></meta></basefont></bgsound></embed></spacer></p></dd></dt></caption></colgroup></tbody></tfoot></thead></address></blockquote></center></dir></div></dl></fieldset></listing></menu></ol></ul></li></nobr></wbr></form></button></marquee></object></html></frameset></head></iframe></image></isindex></noembed></noframes></noscript></optgroup></option></plaintext></textarea>

Trac ticket: TBD

Alternate to #6983 which adds support for all adoption agency algorithm behaviors.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude, Codex
Used for: Fuzz testing.

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[dmsnell]

@sirreal I like moving in_body_any_other_end_tag() to its own method; I recently encountered that in some foreign-element thing too.

would it make sense to do that in a separate PR and then update this so that it’s focused on the adoption-agency algorithm?

[sirreal]

I like moving in_body_any_other_end_tag() to its own method; I recently encountered that in some foreign-element thing too.

Sure, that sounds good to me.