fix(rust, python): pow return type evaluation

[CanglongCl]

fix comment of #12129.

Now dtype of series returned from pow will be evaluate:

If exponent is float, it follows dtype of exponent. Otherwise, it follows dtype of base.

The behavior is the same as pow in python std lib and pow in num_trait.

[codecov]

Codecov Report

Attention: Patch coverage is 96.15385% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.14%. Comparing base (31df06d) to head (9dd04d9).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/pow.rs	94.73%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15506   +/-   ##
=======================================
  Coverage   81.14%   81.14%           
=======================================
  Files        1362     1362           
  Lines      174951   174931   -20     
  Branches     2533     2531    -2     
=======================================
- Hits       141960   141954    -6     
+ Misses      32508    32494   -14     
  Partials      483      483           

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[ritchie46]

Looks good @CanglongCl. Thank you.