Integrate CastColumnExpr into PhysicalExprAdapter #20163

Summary

Wire CastColumnExpr into PhysicalExprAdapter to emit struct-aware cast expressions during physical plan adaptation. Update optimizer and pruning rules as needed.

Scope

Complete the integration of CastColumnExpr into the adapter's rewrite pipeline, ensuring struct fields are correctly cast and metadata is propagated.

Tasks

• Update PhysicalExprAdapter to emit CastColumnExpr (instead of current fallback logic)
• Integrate with schema rewriter refactoring from ISSUE_01
• Update optimizer rules to handle CastColumnExpr (if needed)
• Update partition pruning to account for CastColumnExpr (if needed)
• Add focused behavior tests validating adapter + cast interaction
• Smoke test with struct projection and filtering queries

Files/Modules

• datafusion/physical-expr-adapter/src/schema_rewriter.rs (primary integration point)
• datafusion/optimizer/ (rules that touch expressions)
• datafusion/pruning/ (partition pruning rules)
• Focused integration tests

Key Behaviors

• Struct projection: Correctly adapt struct fields to target schema
• Nullability propagation: Field nullability matches target schema
• Partition pruning: Works correctly with adapted cast expressions
• Plan optimization: Optimizer rules don't break on CastColumnExpr

Acceptance Criteria

✅ Adapter emits CastColumnExpr for schema mismatches
✅ Struct field casting works end-to-end
✅ Optimizer and pruning rules pass all existing tests
✅ New behavior tests validate adapter + cast interaction
✅ Code compiles without warnings