fix: Incorrect mode for sorted input

[orlp]

Fixes the following:

>>> df = pl.DataFrame({"a": [1, 1, 2]})
>>> df.select(pl.col.a.mode())
shape: (1, 1)
┌─────┐
│ a   │
│ --- │
│ i64 │
╞═════╡
│ 1   │
└─────┘
>>> df.set_sorted("a").select(pl.col.a.mode())
shape: (1, 1)
┌─────┐
│ a   │
│ --- │
│ i64 │
╞═════╡
│ 2   │
└─────┘

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.86%. Comparing base (aec911f) to head (bdc29ec).
Report is 34 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-ops/src/chunked_array/mode.rs	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18945      +/-   ##
==========================================
- Coverage   79.86%   79.86%   -0.01%     
==========================================
  Files        1522     1522              
  Lines      206951   206958       +7     
  Branches     2906     2906              
==========================================
+ Hits       165289   165293       +4     
- Misses      41114    41117       +3     
  Partials      548      548              

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