Prepare `PhysicalExprAdapter` internals for the final switch without changing external behavior.

[kosiew]

5. Refactor schema rewriter groundwork

• Goal: Prepare PhysicalExprAdapter internals for the final switch without changing external behavior.

• Scope & Location: datafusion/physical-expr-adapter/src/schema_rewriter.rs (and any helpers it calls).

• Key Tasks:

  • Isolate the structural cleanup already proposed (e.g., factor out helper functions, introduce quick struct compatibility checks) while keeping current cast behavior.
  • Add/adjust tests ensuring no functional change (existing CastExpr path still used).
  • Document new helper responsibilities to aid the next PR.

• Dependencies: Independent of 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 but should land before spec Consolidate TPC-H benchmarks #6 to minimize diff.

• Out of Scope: Introducing CastColumnExpr usage.