fix: Return appropriate data type for time mean and median

[mcrumiller]

Resolves #14470. Part of overall #13599.

This also moves one step closer to deprecating Series.dt.mean and Series.dt.median, as the base Series' mean and median now support time dtypes.

Using the example from the issue, all cases now properly output a duration:

06:00:00
02:00:00
06:00:00
02:00:00
shape: (1, 2)
┌──────────┬──────────┐
│ mean     ┆ median   │
│ ---      ┆ ---      │
│ time     ┆ time     │
╞══════════╪══════════╡
│ 06:00:00 ┆ 02:00:00 │
└──────────┴──────────┘
shape: (1, 2)
┌──────────┬──────────┐
│ mean     ┆ median   │
│ ---      ┆ ---      │
│ time     ┆ time     │
╞══════════╪══════════╡
│ 06:00:00 ┆ 02:00:00 │
└──────────┴──────────┘

[MarcoGorelli]

looks good, just some minor comments

[mcrumiller]

@MarcoGorelli I believe I've resolved all of your issues with minor test improvements:

• Added Duration and Time to the regular Series.mean and Series.median checks (not in the dt namespace). I am not sure if these tests should be moved out of the namespace test, but I think they still logically belong, even if they are not technically part of the namespace anymore. When we get Date in here, we can deprecate the dt.mean and dt.median and, after that, move them over to the regular series tests.
• Parametrized time unit where applicable
• Added a few None cases to existing parametrizations
• Clarified the id of certain parametrizations
• Added an additional aggregate median test to match the existing aggregate mean test.

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 81.16%. Comparing base (dcee934) to head (e2cfc2a).
Report is 4 commits behind head on main.

Files	Patch %	Lines
...s-core/src/frame/group_by/aggregations/dispatch.rs	50.00%	2 Missing ⚠️
py-polars/polars/series/datetime.py	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14471      +/-   ##
==========================================
+ Coverage   81.14%   81.16%   +0.01%     
==========================================
  Files        1363     1368       +5     
  Lines      175282   175417     +135     
  Branches     2527     2527              
==========================================
+ Hits       142236   142379     +143     
+ Misses      32568    32561       -7     
+ Partials      478      477       -1     

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[MarcoGorelli]

minor typing comment, but I think this looks good

thanks for sticking with these!

[MarcoGorelli]

thanks @mcrumiller !