Conditional formatting XML output cannot be roundtripped by other libraries

[wolfiesch]

Description

When umya-spreadsheet writes conditional formatting rules to an .xlsx file, the resulting XML cannot be successfully parsed (roundtripped) by other xlsx libraries such as openpyxl.

Steps to Reproduce

1. Create a new workbook with umya-spreadsheet
2. Add conditional formatting rules (e.g., cell value > 50 → red fill)
3. Save the workbook
4. Open the saved file with openpyxl (Python) → parsing errors

Expected Behavior

The conditional formatting XML written by umya should be valid and parseable by other OOXML-compliant readers.

Actual Behavior

openpyxl fails to parse the conditional formatting rules from the umya-written file. This suggests the XML serialization may not fully conform to the ECMA-376 specification for conditional formatting elements.

Impact

This prevents roundtrip testing of conditional formatting: write with umya → read with openpyxl (oracle) fails, giving umya a 0 score on CF write verification even though the read path works correctly.

Environment

• umya-spreadsheet 2.2.x
• openpyxl 3.1.x (used as verification oracle)
• Discovered via ExcelBench fidelity testing