fix: Fix list.mean and list.median returning Float64 for temporal types

[lukemanley]

closes #17639
closes #19917

[codecov]

Codecov Report

Attention: Patch coverage is 97.36842% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.88%. Comparing base (e8e0295) to head (5655f0b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/schema.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #21144   +/-   ##
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  Coverage   79.88%   79.88%           
=======================================
  Files        1593     1593           
  Lines      227649   227683   +34     
  Branches     2600     2600           
=======================================
+ Hits       181860   181895   +35     
+ Misses      45192    45191    -1     
  Partials      597      597           

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[orlp]

Unfortunately it's not that simple to just cast back to the input type. Take a look at crates/polars-expr/src/reduce/mean.rs and see the corner cases handled there in finish_output. We should probably centralize this logic somewhere...

[lukemanley]

Unfortunately it's not that simple to just cast back to the input type. Take a look at crates/polars-expr/src/reduce/mean.rs and see the corner cases handled there in finish_output. We should probably centralize this logic somewhere...

Thanks @orlp. I've updated to handle those cases although this does not centralize that logic.