fix: Various schema corrections

[ritchie46]

• fix: Window expressions now show proper schema
• fix: Raise when namespace is called in invalid dtypes
• chore: Split up code for maintainability
• perf: Ensure we don't hit the function resolving every time
• perf: Delay alloction

fixes #18468

[ritchie46]

Will try this in a follow up.

[codecov]

Codecov Report

Attention: Patch coverage is 89.38547% with 19 lines in your changes missing coverage. Please review.

Project coverage is 79.87%. Comparing base (8b65fe3) to head (4c337ce).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...s-plan/src/plans/conversion/type_coercion/is_in.rs	84.84%	10 Missing ⚠️
...an/src/plans/conversion/type_coercion/functions.rs	88.88%	5 Missing ⚠️
...tes/polars-plan/src/plans/conversion/expr_to_ir.rs	75.00%	2 Missing ⚠️
...ates/polars-plan/src/plans/conversion/functions.rs	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18474      +/-   ##
==========================================
+ Coverage   79.84%   79.87%   +0.03%     
==========================================
  Files        1498     1501       +3     
  Lines      201933   202032      +99     
  Branches     2867     2868       +1     
==========================================
+ Hits       161228   161375     +147     
+ Misses      40159    40110      -49     
- Partials      546      547       +1     

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