fix: Raise instead of panicking when casting a Series to a Struct with the wrong number of fields

[etiennebacher]

Fixes #14083

The current behavior when casting a single series to a struct with multiple fields is to fill with nulls:

import polars as pl
print(
  pl.Series([1], dtype=pl.Int32).cast(pl.Struct({"a": pl.UInt8, "b": pl.UInt8}))
)
# shape: (1,)
# Series: '' [struct[2]]
# [
#        {1,null}
# ]

With this PR, this now throws an error but I don't know if that's considered a breaking change or a fix (the current behavior is untested).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.87%. Comparing base (515c1b8) to head (1d954a0).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21213      +/-   ##
==========================================
+ Coverage   79.80%   79.87%   +0.07%     
==========================================
  Files        1596     1596              
  Lines      228468   228469       +1     
  Branches     2607     2607              
==========================================
+ Hits       182318   182484     +166     
+ Misses      45554    45389     -165     
  Partials      596      596              

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