Ensure plans containing `CastColumnExpr` round-trip through protobuf for distributed execution.

[kosiew]

3. Add serialization/deserialization support

• Goal: Ensure plans containing CastColumnExpr round-trip through protobuf for distributed execution.

• Scope & Location: datafusion/physical-plan protobuf definitions and converters; round-trip tests under datafusion/physical-plan or related crates.

• Key Tasks:

  • Extend protobuf schema with a message for CastColumnExpr carrying input/target field descriptors.
  • Update Rust proto builders to emit/consume the new message, mapping back to the expression type.
  • Add/extend round-trip tests to cover CastColumnExpr inside representative plans.
  • Document any compatibility considerations (e.g., versioning, defaults).

• Dependencies: Requires CastColumnExpr definition (spec Add a dedicated physical expression for struct-aware casting that mirrors the existing cast_column helper semantics. #17760).

• Out of Scope: Planner rewrites or optimizer behavior.