fix: Raise on unsupported fill strategy dtype

[ritchie46]

fixes #15782 fixes #17830

We should expand functionality here.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.47%. Comparing base (0c2cfae) to head (aff9a9a).

Files	Patch %	Lines
...tes/polars-core/src/chunked_array/ops/fill_null.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17837      +/-   ##
==========================================
- Coverage   80.50%   80.47%   -0.04%     
==========================================
  Files        1504     1504              
  Lines      197152   197153       +1     
  Branches     2805     2805              
==========================================
- Hits       158722   158653      -69     
- Misses      37909    37979      +70     
  Partials      521      521              

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