fix(html): stop a header-only rowspan table from crashing the backend

[chuenchen309]

An HTML table whose only row is a th with rowspan — no body rows for it to span into — crashes the backend:

HTMLDocumentBackend(...).convert()   # <table><tr><th rowspan="2">h</th></tr></table>
# IndexError: list index out of range

get_html_table_row_col treats an all-header row that uses rowspan as a spanning header and doesn't count it, so for a table with only such a row num_rows is 0 and the grid is empty. The cell-placement while loop then reads grid[row_idx + start_row_span][col_idx] without a row bound and goes out of range — while the write just below it already guards row_idx + r < num_rows.

The fix adds the same row bound to the read. The cell is kept and the whole document no longer aborts on this one table. Normal tables (including header rows that span into real body rows) are unaffected — this only adds a bounds check.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

test_table_header_rowspan_without_body_does_not_crash converts that table and asserts it doesn't crash and keeps the cell; it fails before this change (IndexError) and passes after. The full tests/test_backend_html.py stays green (37 passed); ruff check / ruff format pass.

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Disclosure: this contribution is fully AI-authored and autonomous (Claude Code, acting on this account). An AI found the bug, wrote and ran the repro and the test, and wrote this description; the human account holder reviews every change and is accountable for it. The verification above is real and re-runnable from the diff. If this isn't the kind of contribution you want, say so and I'll close it.

[github-actions]

❌ DCO Check Failed

Hi @chuenchen309, your pull request has failed the Developer Certificate of Origin (DCO) check.

This repository supports remediation commits, so you can fix this without rewriting history — but you must follow the required message format.

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I, chuenchen309 <48723787+chuenchen309@users.noreply.github.com>, hereby add my Signed-off-by to this commit: 4f6b2ffdea5817f5ccc3729444bc1ee5a265b8b0"
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• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[dolfim-ibm]

@chuenchen309 thanks for the fix, can you please fix the sign-off of your contribution?

See the message at #3827 (comment)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[ceberam]

Good catch and effective fix!
Just a small comment, but I'll approve once the DCO is fixed.

[ceberam]

I would remove these inline comments. The commit content, the PR description and the issue description should be enough to track the background of the fix. You can simply add 1 line description as a docstrings subject of the test.