chore: Add proper tests for row encoding

[coastalwhite]

This adds the following method to be used internally to create tests.

• pl.DataFrame._row_encode()
• pl.Series._row_decode()

With this I added a bunch of tests to see the current state of row encoding.

[codecov]

Codecov Report

Attention: Patch coverage is 96.96970% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.40%. Comparing base (420c0d9) to head (f195eb2).
Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/dataframe/frame.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19843      +/-   ##
==========================================
- Coverage   79.55%   79.40%   -0.16%     
==========================================
  Files        1544     1550       +6     
  Lines      213240   214840    +1600     
  Branches     2441     2460      +19     
==========================================
+ Hits       169645   170583     +938     
- Misses      43046    43698     +652     
- Partials      549      559      +10     

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