2. Teach physical-expression utilities about CastColumnExpr

• Goal: Ensure all helper subsystems that reason about expressions understand the new node before other components depend on it.

• Scope & Location: datafusion/physical-expr-common, physical-expr optimizers/utilities, plus associated tests.

• Key Tasks:

  • Extend equivalence tracking to treat CastColumnExpr similarly to other projection expressions when comparing canonical forms.
  • Update interval propagation / ordering analysis to either unwrap or conservatively handle CastColumnExpr.
  • Teach cast-unwrapping simplifications (strip_aliases, strip_nullability_casts, etc.) to recognize when a CastColumnExpr can be bypassed (or confirm it must stay).
  • Add regression tests for each utility verifying expected behavior with the new node.

• Dependencies: Builds on spec Add a dedicated physical expression for struct-aware casting that mirrors the existing cast_column helper semantics. #17760 (expression must exist).

• Out of Scope: Planner adapters and serialization changes.