Add schema-aware optimizer child rewrites

[sakshichitnis27]

Fixes #23666

Summary

• Add an optimizer-local helper for schema-aware direct-child rewrites.
• Recompute a parent schema only when a rewritten child’s schema changes.
• Migrate EliminateCrossJoin to use the helper.
• Preserve existing in-place optimizer schema protection.
• Add tests for schema-changing and schema-preserving child rewrites.

This is an internal optimizer refactor and does not change SQL-visible behavior.

Testing

• cargo fmt --all --check
• cargo test -p datafusion-optimizer

The optimizer tests passed, including unit tests, integration tests, and doctests.
cargo clippy is currently blocked by an unrelated existing dead_code error in
datafusion-physical-expr.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.58621% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.75%. Comparing base (2e3626e) to head (6f918d0).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/optimizer/src/optimizer.rs	75.92%	0 Missing and 13 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23686      +/-   ##
==========================================
- Coverage   80.75%   80.75%   -0.01%     
==========================================
  Files        1096     1096              
  Lines      373588   373642      +54     
  Branches   373588   373642      +54     
==========================================
+ Hits       301691   301719      +28     
- Misses      53896    53902       +6     
- Partials    18001    18021      +20     

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[kosiew]

@sakshichitnis27
Thanks for working on this.

Could we consider migrating remaining owned-child rewrite path in datafusion/optimizer/src/optimize_projections/mod.rs' fn optimize_projections

    // Rewrite children of the plan
    let transformed_plan = plan.map_children(|child| {

to map_children_recompute_schema_if_needed in a follow-up, while keeping the separate optimize_subqueries handling?

It currently uses a manual map_children followed by an unconditional recompute_schema() whenever a transformation occurs. Using the new helper would keep the child-schema cache invariant more consistently owned by one API, and it may also avoid unnecessary recomputes when a child's internals change without affecting its schema.