refactor(rust): Fix mean reduction in new-streaming

[orlp]

Now using _sum_as_f64 and making sure to get the dtype from the input rather than the output (which is different for Dates).

[codecov]

Codecov Report

Attention: Patch coverage is 12.00000% with 44 lines in your changes missing coverage. Please review.

Project coverage is 79.84%. Comparing base (c85b338) to head (9784d2b).
Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/reduce/convert.rs	0.00%	17 Missing ⚠️
.../polars-core/src/series/implementations/boolean.rs	0.00%	3 Missing ⚠️
...tes/polars-core/src/series/implementations/date.rs	0.00%	3 Missing ⚠️
...polars-core/src/series/implementations/datetime.rs	0.00%	3 Missing ⚠️
.../polars-core/src/series/implementations/decimal.rs	0.00%	3 Missing ⚠️
...polars-core/src/series/implementations/duration.rs	0.00%	3 Missing ⚠️
...s/polars-core/src/series/implementations/floats.rs	0.00%	3 Missing ⚠️
...ates/polars-core/src/series/implementations/mod.rs	0.00%	3 Missing ⚠️
...tes/polars-core/src/series/implementations/time.rs	0.00%	3 Missing ⚠️
crates/polars-core/src/series/series_trait.rs	0.00%	2 Missing ⚠️
... and 1 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18572   +/-   ##
=======================================
  Coverage   79.83%   79.84%           
=======================================
  Files        1501     1501           
  Lines      201736   201764   +28     
  Branches     2869     2869           
=======================================
+ Hits       161059   161097   +38     
+ Misses      40131    40121   -10     
  Partials      546      546           

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