fix: Categorical min/max returning String dtype rather than Categorical

[lukemanley]

Min/max aggregations on categorical dtypes currently return string dtype. This PR changes the output to maintain the categorical dtype as is done for enums. This also aligns the output with the expected schema which was misaligned.

import polars as pl

lf = pl.LazyFrame(
    {
        "a": ["foo", "bar"],
        "b": ["foo", "bar"],
        "c": ["foo", "bar"],
    },
    schema={
        "a": pl.Categorical("physical"),
        "b": pl.Categorical("lexical"),
        "c": pl.Enum(["foo", "bar"]),
    },
)

q = lf.select(pl.all().max())

# expected schema is already correct:
print(q.collect_schema())
# Schema({'a': Categorical(ordering='physical'), 'b': Categorical(ordering='lexical'), 'c': Enum(categories=['foo', 'bar'])})

print(q.collect())

# BEFORE:

# ┌─────┬─────┬──────┐
# │ a   ┆ b   ┆ c    │
# │ --- ┆ --- ┆ ---  │
# │ str ┆ str ┆ enum │
# ╞═════╪═════╪══════╡
# │ foo ┆ bar ┆ foo  │
# └─────┴─────┴──────┘

# AFTER:

# ┌─────┬─────┬──────┐
# │ a   ┆ b   ┆ c    │
# │ --- ┆ --- ┆ ---  │
# │ cat ┆ cat ┆ enum │
# ╞═════╪═════╪══════╡
# │ foo ┆ bar ┆ foo  │
# └─────┴─────┴──────┘

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.89%. Comparing base (947dc07) to head (1e288f9).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
...polars-core/src/chunked_array/ops/aggregate/mod.rs	95.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21232      +/-   ##
==========================================
+ Coverage   79.82%   79.89%   +0.07%     
==========================================
  Files        1596     1596              
  Lines      228517   228552      +35     
  Branches     2608     2608              
==========================================
+ Hits       182413   182604     +191     
+ Misses      45508    45352     -156     
  Partials      596      596              

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[ritchie46]

Nice, the failures shuold be fixed on main. Can you rebase?

[ritchie46]

Can we also check that the schema contains categoricals and enums?

[lukemanley]

added more explicit check

[ritchie46]

Thanks!