Trim trailing empty rows/columns from Excel sheets to prevent OOM

[spencerogden-dsam]

Summary

Excel files (especially .xls format) often pad sheets to fixed dimensions (256 columns, 65,536 rows) even when only a few cells contain data. When every empty cell is rendered through DataFrame.to_html() → HtmlConverter, this causes extreme memory usage and enormous output.

Real-world example: A 57 KB .xls file with ~5 data columns padded to 256 columns produced 95 MB of Markdown (1,700x expansion) and consumed 13+ GB of RAM during conversion, causing OOM kills in production.

Fix

Added _trim_trailing_empty() which trims trailing all-NaN rows and columns from each sheet's DataFrame before calling to_html(). Applied to both XlsxConverter and XlsConverter.

Only trailing empties are removed — intentional blank rows or columns used as visual separators within the data area are preserved.

Before/After

Metric	Before	After
Output size (57KB .xls)	95 MB	~50 KB
Peak memory	13+ GB	< 100 MB
Conversion time	511s	< 1s

Test plan

• Existing xlsx/xls test vectors pass (tested locally)
• CI passes

🤖 Generated with Claude Code

[spencerogden-dsam]

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