fix(python): Ensure default values are included when saving/restoring the current Config state

[alexander-beedie]

Found a bug with Config object load and save methods; configuration options that had not been explicitly set to a non-default value were not being properly captured or restored.

Fixed this and added suitable test coverage.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.33%. Comparing base (8c19d18) to head (e6e4ede).
Report is 1 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/config.py	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18151      +/-   ##
==========================================
- Coverage   80.34%   80.33%   -0.01%     
==========================================
  Files        1496     1496              
  Lines      197809   197814       +5     
  Branches     2822     2824       +2     
==========================================
- Hits       158934   158923      -11     
- Misses      38353    38369      +16     
  Partials      522      522              

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