refactor: Use UnionArgs for DSL side

[ritchie46]

Ensure we don't expose IR state to the DSL.

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 80.91%. Comparing base (b7b3da6) to head (4d52575).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/polars-plan/src/logical_plan/tree_format.rs	0.00%	8 Missing ⚠️
crates/polars-plan/src/logical_plan/options.rs	95.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16017   +/-   ##
=======================================
  Coverage   80.91%   80.91%           
=======================================
  Files        1385     1385           
  Lines      178225   178245   +20     
  Branches     3050     3050           
=======================================
+ Hits       144213   144231   +18     
+ Misses      33526    33525    -1     
- Partials      486      489    +3     

Flag	Coverage Δ
python	74.40% <85.00%> (+<0.01%)	⬆️
rust	78.13% <85.00%> (+<0.01%)	⬆️

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