fix: Fix Expr.over applying scale incorrectly for Decimal types

[lukemanley]

fixes #21096

import polars as pl

df = pl.Series("x", [1, 2], pl.Decimal(scale=2)).to_frame()
df.select(pl.col("x").min().over("x"))

Before:

shape: (2, 1)
┌──────────────┐
│ x            │
│ ---          │
│ decimal[*,2] │
╞══════════════╡
│ 100.00       │
│ 200.00       │
└──────────────┘

After:

shape: (2, 1)
┌──────────────┐
│ x            │
│ ---          │
│ decimal[*,2] │
╞══════════════╡
│ 1.00         │
│ 2.00         │
└──────────────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.87%. Comparing base (1aab51c) to head (42b7665).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #21140   +/-   ##
=======================================
  Coverage   79.87%   79.87%           
=======================================
  Files        1593     1593           
  Lines      227467   227467           
  Branches     2600     2600           
=======================================
+ Hits       181686   181689    +3     
+ Misses      45184    45181    -3     
  Partials      597      597           

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