Prevent XLS precision loss for large integers

[ViceSaber]

Summary

• prevent precision loss when exporting large integers to XLS format
• integers exceeding XLS float precision (2^53 - 1) are now stored as text strings

Problem

XLS (Excel 97-2003) stores numeric values as 64-bit IEEE 754 floats with a 53-bit mantissa. Integers larger than 9,007,199,254,740,991 (2^53 - 1) lose precision when stored as numbers. For example, 568883383628111872 becomes 568883383628112000.

Fix

In dset_sheet, detect integers that exceed the float precision limit and convert them to strings before writing. Small integers continue to be stored as numbers for normal behavior.

Testing

Verified that:

• Large integers (e.g., 568883383628111872) are stored as text (cell type 1) and read back with full precision
• Small integers (e.g., 42) continue to be stored as numbers (cell type 2)
• Boolean values are unaffected (not coerced to string)

Fixes #197

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.12%. Comparing base (7d6c58a) to head (bff868a).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/tablib/formats/_xls.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #637      +/-   ##
==========================================
- Coverage   93.14%   93.12%   -0.03%     
==========================================
  Files          29       29              
  Lines        3226     3229       +3     
==========================================
+ Hits         3005     3007       +2     
- Misses        221      222       +1     

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[hugovk]

Thanks for the PR, please add tests.

I've just merged a fix for the lint teyit failure, please also rebase/merge main to include that.

[claudep]

Another approach would have been to document that the XLS format is not adequate to store large integers. Is that format not on a disappearing trend?

[hugovk]

I'm fine with either approach.

This fix seems simple enough, but I expect XLS use is very low, and outputting XLS even lower.

A docs change is safer, and we've not exactly had a huge number of requests for this.