fix: preserve row_number columns in CSVToDocument

[LimbC-C]

Related Issues

Fixes #12787.

Proposed Changes:

Preserve a CSV column named row_number in CSVToDocument row mode. Previously the generated row index overwrote that column after metadata collision handling, silently losing source record identifiers.

Reserve row_number before merging CSV columns so the existing csv_ prefix and numeric suffix handling preserves the original value. Add regressions for ordinary and occupied-prefix metadata, plus a control selecting row_number as the content column. Update the component docstring, current/versioned docs, and release note.

How did you test it?

• Before the fix: both preservation regressions fail with the expected missing-metadata assertions; the content-column control passes.
• After the fix: all 20 tests in test/components/converters/test_csv_to_document.py pass; CSV converter statement coverage is 95%.
• Focused mypy: no issues found in the source and test files.
• All applicable pre-commit hooks and git diff --cached --check pass.

Validation ran via Hatch on Windows / Python 3.12.14 using local ByteStreams. Pytest emitted two NumPy-reload warnings from Haystack imports. No live model, external service, or full integration suite was run.

Notes for the reviewer

Generated, reviewed by the coding agent, and tested with Codex at the account owner's request. No human review is claimed.

The change is independent of #12772, which fixes textual NA markers in CSVDocumentCleaner. The converter, its tests, and contribution guidelines were unchanged between the local base and current upstream at verification.

Checklist

• Contribution guidelines and code of conduct read by Codex.
• Reproduction and test results documented in CSVToDocument row mode overwrites a CSV column named row_number #12787.
• Regression tests, docstring, component documentation and release note included.
• Conventional commit title used.
• Applicable pre-commit hooks passed locally.

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Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/components/converters
csv.py
Project Total

This report was generated by python-coverage-comment-action

[anakin87]

I pushed a few simplifications.

Looks good!