fix: Don't reuse cached return_dtype for expanded map expressions

[coastalwhite]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.60%. Comparing base (edbb8b7) to head (7a76ecc).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/dsl/expr/expr_dyn_fn.rs	50.00%	3 Missing ⚠️
...n/src/plans/conversion/dsl_to_ir/expr_expansion.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24010      +/-   ##
==========================================
+ Coverage   81.58%   81.60%   +0.01%     
==========================================
  Files        1675     1675              
  Lines      225708   225725      +17     
  Branches     2897     2897              
==========================================
+ Hits       184138   184195      +57     
+ Misses      40842    40802      -40     
  Partials      728      728              

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