fix(html): enable huge_tree on HTMLParser so deeply nested HTML partitions

[CrepuscularIRIS]

Summary

Fixes #4289.

partition_html silently returned zero elements for HTML documents whose DOM depth exceeded lxml's default huge_tree=False depth limit (~256 levels). The module-level etree.HTMLParser in unstructured/partition/html/parser.py was constructed without huge_tree, so etree.fromstring dropped the subtree beyond the limit and the downstream flow saw an empty document.

This PR flips the kwarg to huge_tree=True on that single shared parser and adds a regression test.

Root Cause

html_parser = etree.HTMLParser(remove_comments=True) inherits lxml's default huge_tree=False, which caps tree depth / node text size. When etree.fromstring(html_text, html_parser) in partition.py:258 hits that cap, the subtree past the cap is dropped, producing an empty Flow root — so partition_html yields [] for large/deeply-nested HTML.

Changes

• unstructured/partition/html/parser.py: pass huge_tree=True to the shared etree.HTMLParser (1 line).
• test_unstructured/partition/html/test_partition.py: add test_partition_html_parses_deeply_nested_html, which builds a 260-level <div> wrapper around a <p> and asserts the inner text is recovered.
• CHANGELOG.md: new 0.22.22 section under ### Fixes.
• unstructured/__version__.py: bump 0.22.21 → 0.22.22.

Testing

• New regression test passes with the fix and fails without it (verified by reverting the kwarg locally).
• test_unstructured/partition/html/ — 312/312 passing.
• Broader run test_unstructured/partition/html/ + partition/test_xml.py + partition/test_text.py — 390/390 passing.

pytest test_unstructured/partition/html/test_partition.py::test_partition_html_parses_deeply_nested_html -v

Notes

• huge_tree=True on HTMLParser relaxes lxml's tree-size / depth safeguards. It does not enable entity expansion (that flag is resolve_entities, which remains off by default), so no new XXE surface is introduced.
• Minimal fix — no other behavior or API is changed. Only production change is one kwarg; tree is still shared across all callers.
• Happy to switch to a per-call huge_tree opt-in if preferred, but the shared-parser approach matches what the issue and the prior self-closed PR fix: enable huge_tree for HTMLParser to handle large documents #4306 proposed.

[cragwolfe]

ref: https://lxml.de/FAQ.html

There is a safety reason why this should not be the default. I think the default should remain False. @CrepuscularIRIS , can you amend by having this PR to instead have the behavior overridable by env var (with default False)?

[CrepuscularIRIS]

Thanks @cragwolfe — fully agree on the safety concern. I've pushed fe6216d which:

• Restores huge_tree=False as the default
• Adds opt-in via the UNSTRUCTURED_HTML_HUGE_TREE env var (accepts 1/true/yes)
• Updates the changelog to document the env var and link the lxml security note
• Adds a second test asserting the default-off behavior so the safety posture is enforced going forward

Both tests pass locally; full test_partition.py suite still passes (105/105).