fix(html): extract <details>/<summary> content instead of discarding it

[r0h1tb]

Problem

<details> and <summary> were both mapped to RemovedBlock, so partition_html() discarded the whole subtree. On the FAQ page from the issue, every question and answer disappears and the caller gets a shorter element list rather than an error:

>>> partition_html(text="<h1>Support FAQ</h1>"
...   "<details><summary>Which Bluetooth profiles are supported?</summary>"
...   "<p>Your device supports A2DP and HFP profiles.</p></details>")
[<Title 'Support FAQ'>]          # both the question and the answer are gone

Fix

<details> is an ordinary block container, so it becomes Flow, like <div>.

<summary> becomes Heading. The spec calls it "a summary, caption, or legend for the rest of the contents", the issue asks for the questions as titles, and it is what makes the fix useful downstream — chunk_by_title() opens a new section at each Title, so a question stays attached to its own answer instead of blending into the next entry.

<summary> was also listed twice, under both phrasing and the removed-form group; there is now one mapping.

Scope

Only these two tags. <dl>, <figure> and the form/nav tags stay in RemovedBlock:

• <figure>/<figcaption> removal is asserted by DescribeRemovedBlock.it_is_skipped_during_parsing, so it is a deliberate decision, not an oversight.
• <dl>/<dt>/<dd> also drop their content, and ListBlock's docstring already says "maybe a <dl> element at some point". That looks worth doing, but reclassifying it changes the ancestor chain that _category_depth walks (there is a parametrised case at test_parser.py:298 relying on <dl><dd> nesting), so it belongs in its own PR rather than riding along here.

Tests

Four tests in test_unstructured/partition/html/test_partition.py. With the parser change reverted on this branch:

FAILED test_partition_html_extracts_details_and_summary_content        - assert [] == [(...5 elements)]
FAILED test_partition_html_makes_a_summary_start_its_own_chunk_section
FAILED test_partition_html_extracts_a_details_block_that_has_no_summary
FAILED test_partition_html_extracts_nested_details_blocks

Also checked by hand and behaving: unclosed <details>, <summary> not the first child, two <summary> siblings, empty <summary>, <summary> outside any <details>, attributes on <details>, inline markup inside <summary>, <details> inside a table cell and inside a list item, 20-deep nesting, and a JSON round-trip.

Suite before 146 failed / 2531 passed, after 146 failed / 2535 passed — the same 146 IDs, all pre-existing here (pandoc and the ML extras aren't installed locally). 14 modules that cannot be collected without those extras were excluded from both runs.

Note: this bumps to 0.26.4, as does #4449 — whichever lands second needs the version and changelog re-pointed.

[cubic-dev-ai]

No issues found across 4 files

<sub>Shadow auto-approve: would auto-approve. Bug fix in HTML parsing: previously dropped <details>/<summary> content is now extracted, with tests pinned; low-risk mapping change.

Re-trigger cubic</sub>