Update pruning to handle `CastColumnExpr`

[kosiew]

4. Update pruning to handle CastColumnExpr

• Goal: Preserve statistics-based pruning when predicates include struct-aware casts.

• Scope & Location: datafusion/physical-optimizer/pruning and its tests.

• Key Tasks:

  • Modify rewrite_expr_to_prunable (and related helpers) to unwrap or appropriately interpret CastColumnExpr.
  • Ensure predicate evaluation during pruning handles missing fields/nullable differences correctly.
  • Add regression tests covering struct predicates before/after pruning to confirm behavior is unchanged or improved.

• Dependencies: 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 should be merged; pruning relies on expression and serialization support already in place.

• Out of Scope: Schema adapter rewrites or broader optimizer refactors.