Switch adapter to `CastColumnExpr` and add behavior tests

[kosiew]

6. Switch adapter to CastColumnExpr and add behavior tests

• Goal: Make PhysicalExprAdapter reuse the struct-aware casting path for schema mismatches and validate behavior end-to-end.

• Scope & Location: schema_rewriter.rs, adapter integration tests, potentially shared test utilities.

• Key Tasks:

  • Replace the fallback CastExpr creation with CastColumnExpr instantiation when rewriting columns.
  • update the existing CastExpr to handle Fields in place of CastColumnExpr - discussion
  • Ensure new quick compatibility checks determine when no cast is necessary.
  • Add integration-style tests covering predicates with missing struct fields, differing nullability, and nested structs to confirm correct coercion.
  • Update any documentation or comments referencing the old behavior.

• Dependencies: Requires specs Add a dedicated physical expression for struct-aware casting that mirrors the existing cast_column helper semantics. #17760, Teach physical-expression utilities about CastColumnExpr #17761, Ensure plans containing CastColumnExpr round-trip through protobuf for distributed execution. #17762, Update pruning to handle CastColumnExpr #17763, Prepare PhysicalExprAdapter internals for the final switch without changing external behavior. #17764 to be merged (expression, utilities, serialization, pruning, and refactor).

• Out of Scope: Additional optimizer/planner features beyond the adapter change.