fix: Make struct.with_fields data model coherent

[kdn36]

fixes #24687
fixes #24568
fixes #18461

This PR introduces changes in the DSL, IR and Physical engines (both in-memory and streaming) for struct.with_fields(), such that its data model is coherent.

Pending performance regression analysis.

[codecov]

Codecov Report

❌ Patch coverage is 84.65753% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.69%. Comparing base (8ab1657) to head (fd46ea3).

Files with missing lines	Patch %	Lines
crates/polars-expr/src/expressions/field.rs	77.50%	9 Missing ⚠️
crates/polars-plan/src/plans/aexpr/traverse.rs	77.77%	8 Missing ⚠️
crates/polars-plan/src/dsl/expr/mod.rs	14.28%	6 Missing ⚠️
...ates/polars-stream/src/physical_plan/lower_expr.rs	73.91%	6 Missing ⚠️
crates/polars-expr/src/expressions/structeval.rs	93.90%	5 Missing ⚠️
.../polars-stream/src/physical_plan/lower_group_by.rs	42.85%	4 Missing ⚠️
crates/polars-plan/src/dsl/format.rs	0.00%	3 Missing ⚠️
crates/polars-plan/src/plans/aexpr/equality.rs	0.00%	2 Missing ⚠️
...rs-plan/src/plans/aexpr/predicates/skip_batches.rs	0.00%	2 Missing ⚠️
...n/src/plans/conversion/dsl_to_ir/expr_expansion.rs	93.33%	2 Missing ⚠️
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25610      +/-   ##
==========================================
+ Coverage   79.58%   79.69%   +0.11%     
==========================================
  Files        1743     1745       +2     
  Lines      240439   240720     +281     
  Branches     3038     3038              
==========================================
+ Hits       191347   191838     +491     
+ Misses      48310    48100     -210     
  Partials      782      782              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.