fix: Fix DataFrame.min/max for decimals

[petrosbar]

Now that decimals can be sorted (#14649), is_ord can be true for DataType::Decimal. This will in turn fix the following:

import polars as pl
from decimal import Decimal as D

pl.DataFrame({"a": [D("1.0"), D("2.0")]}).max()

output:

shape: (1, 1)
┌──────────────┐
│ a            │
│ ---          │
│ decimal[*,1] │
╞══════════════╡
│ null         │
└──────────────┘

expected:

shape: (1, 1)
┌──────────────┐
│ a            │
│ ---          │
│ decimal[*,1] │
╞══════════════╡
│ 2            │
└──────────────┘

Similar to #14652

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.01%. Comparing base (4e521e8) to head (b2fc8b6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14890      +/-   ##
==========================================
- Coverage   81.01%   81.01%   -0.01%     
==========================================
  Files        1331     1331              
  Lines      172772   172773       +1     
  Branches     2455     2455              
==========================================
- Hits       139973   139966       -7     
- Misses      32332    32339       +7     
- Partials      467      468       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[stinodego]

Thanks for the fix!