fix(markdown_parser): emit MdIndentTokenList for headers inside list items

[jfmcdowell]

Note

This PR was created with AI assistance (Claude Code).

Summary

Fixes #9771.

Emit the required MdIndentTokenList when parsing ATX headers in list item content so MdHeader matches the grammar-defined slot layout.

Test Plan

• just test-crate biome_markdown_parser — all passed
• just test-markdown-conformance — all passed
• just test-crate biome_markdown_formatter — all passed
• just f
• just l

Docs

N/A.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 9b88ca3

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[codspeed-hq]

Merging this PR will not alter performance

✅ 28 untouched benchmarks
⏩ 228 skipped benchmarks¹

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_{Comparing jfmcdowell:fix/md-header-indent-in-list (9b88ca3) with main (552a412)²}

Footnotes

1. 228 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (ac93071) during the generation of this report, so 552a412 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[coderabbitai]

Walkthrough

This PR fixes the parser's handling of ATX headings within list items by emitting an explicit MdIndentTokenList CST node before parsing the heading content. The fix ensures that header nodes maintain a standard slot layout regardless of context. A test fixture file with various heading-in-list combinations has been added to expand parser test coverage.

Possibly related PRs

• refactor(markdown): emit continuation indent as structural CST node #9737: Also modifies the parser to emit explicit indent CST nodes via emit_line_indent, affecting CST shape for list-related constructs.
• fix(markdown): preserve nested list indent tokens #9717: Modifies list parsing in the same file to change how indentation tokens are emitted and preserved in list-item contexts.

Suggested reviewers

• dyc3
• ematipico

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarises the main change: emitting MdIndentTokenList for headers inside list items, which is the core fix for issue #9771.
Linked Issues check	✅ Passed	The PR successfully addresses the coding requirement from #9771 by emitting the missing MdIndentTokenList in parse_first_line_atx_heading when parsing headers in list items, ensuring well-formed MdHeader nodes.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the parser issue: modifying the heading parser logic and adding test fixtures for headers inside lists. No unrelated changes detected.
Description check	✅ Passed	The pull request description clearly relates to the changeset, explaining the fix for issue #9771 and detailing the parser changes with test results.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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