feat: Catch use of 'polars' in to_string for non-Duration dtypes and raise an informative error

[alexander-beedie]

Closes #19970.

Makes sense to add a simple dtype check for use of expr.dt.to_string("polars") with non-Duration dtypes as it is a special-case.

[codecov]

Codecov Report

Attention: Patch coverage is 73.07692% with 7 lines in your changes missing coverage. Please review.

Project coverage is 79.48%. Comparing base (fdf9751) to head (8921070).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...lars-core/src/chunked_array/temporal/conversion.rs	68.18%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19977      +/-   ##
==========================================
- Coverage   79.49%   79.48%   -0.01%     
==========================================
  Files        1555     1556       +1     
  Lines      216377   216397      +20     
  Branches     2456     2456              
==========================================
+ Hits       172002   172006       +4     
- Misses      43817    43833      +16     
  Partials      558      558              

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