Add tests directory to source distribution

[yohaann196]

Fixes the issue where the tests directory was missing from the 3.1.0 source distribution. I added "tests" to the source-include list in pyproject.toml since this project uses uv_build as the build backend. Verified locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (36ec238) to head (581cf83).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #285   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          130       130           
  Branches        14        14           
=========================================
  Hits           130       130           

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