fix(xlsx): preserve entered float digits by setting float_format=str (#2532) - #2656

Closes #2532

Summary of Changes

• Passes float_format=str to DataFrame.to_html() in both XlsxConverter and XlsConverter.
• Prevents pandas' default 6-digit scientific notation from mangling raw float cell values (e.g., losing fractional digits on large numbers or converting 0.1 to 1.000000e-01).
• Adds regression unit test verifying that precise float values like 123456789.123 and 0.1 round-trip without scientific notation truncation.