HTML Api: Fix out of bounds string access

[sirreal]

Trac ticket: https://core.trac.wordpress.org/ticket/60108

The HTML API Tag Processor may attempt to perform out of range string index access, which may manifest as:

ValueError: strpos(): Argument #3 ($offset) must be contained in argument #1 ($haystack)

• Add tests (failing on current trunk) to Tag Processor test suite that demonstrate the issue.
• Fix out of range checks.

This PR also reuses a $doc_length variable. There's a strlen( $html ) performed at the top of the function and the result can be reused instead of calling strlen repeatedly.

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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.

[github-actions]

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[dmsnell]

Rebuilt on to #5725 in aa24abd

[dmsnell]

@sirreal I've cherry-picked my rebuilt change in 9e7167a onto #5725. If you want we can re-target this PR against that branch or close it out. I'm worried about losing the change in the stacked PRs if we merge it first, but if you prefer that we can do that and I'll rebuild avoid-parsing-incomplete-tokens on top of it.

[ockham]

Committed to Core as part of https://core.trac.wordpress.org/changeset/57211 (see #5793 (comment)).