feat: Always resolve dynamic types in schema

[ritchie46]

Always ensure dynamic types are materialized in schema as that is what will be produced.

Also fixes #20355

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 8 lines in your changes missing coverage. Please review.

Project coverage is 78.98%. Comparing base (68eec85) to head (32936fc).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ates/polars-stream/src/physical_plan/lower_expr.rs	0.00%	5 Missing ⚠️
crates/polars-core/src/datatypes/dtype.rs	92.85%	1 Missing ⚠️
crates/polars-expr/src/reduce/convert.rs	0.00%	1 Missing ⚠️
...ates/polars-plan/src/plans/conversion/dsl_to_ir.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20406      +/-   ##
==========================================
+ Coverage   78.97%   78.98%   +0.01%     
==========================================
  Files        1562     1562              
  Lines      220078   220111      +33     
  Branches     2480     2486       +6     
==========================================
+ Hits       173802   173862      +60     
+ Misses      45705    45676      -29     
- Partials      571      573       +2     

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