refactor: port HashExpr proto hooks

[nanookclaw]

Which issue does this PR close?

• Closes Port HashExpr to use try_to_proto / try_from_proto #22432.

Rationale for this change

HashExpr is part of the physical expression proto cleanup tracked by #22418. Its protobuf serialization should live with the expression implementation instead of in the central proto downcast chains, matching the hook pattern added in #21929.

What changes are included in this PR?

This adds HashExpr's PhysicalExpr::try_to_proto override and a feature-gated inherent HashExpr::try_from_proto. The existing PhysicalHashExprNode wire shape is preserved, including expr_id: None, while datafusion-proto now routes ExprType::HashExpr decoding through the hook and no longer has a central HashExpr serialization arm. datafusion-physical-plan now exposes a proto feature so the hook code only compiles when proto support is requested.

Are these changes tested?

Yes. Added focused direct hook tests for encoding, decoding, and rejecting a wrong expr_type.

Commands run:

• cargo fmt --all -- --check
• cargo check -p datafusion-physical-plan
• cargo test -p datafusion-physical-plan --features proto hash_expr_try
• cargo test -p datafusion-proto --test proto_integration cases::roundtrip_physical_plan::roundtrip_hash_expr
• cargo check -p datafusion-proto
• cargo clippy -p datafusion-physical-plan --features proto --all-targets -- -D warnings
• cargo clippy -p datafusion-proto --all-targets -- -D warnings

Are there any user-facing changes?

No. This preserves the existing protobuf representation.

[adriangb]

@nanookclaw could you rebase and resolve conflicts please?

[nanookclaw]

Rebased and resolved the conflicts. The new head is 768f68d50.

Validation run after the rebase:

• cargo fmt --check -- datafusion/proto/src/physical_plan/from_proto.rs datafusion/proto/src/physical_plan/to_proto.rs
• cargo test -p datafusion-physical-plan hash_expr_try --features proto --lib → 3 passed
• cargo check -p datafusion-proto
• git diff --check upstream/main...HEAD

[nanookclaw]

Rebased fix/hash-expr-proto-hooks onto current main and force-pushed head 20c2832988511969334e9ec4b189e1952365c3d3.

Conflict resolution kept upstream's HashTableLookupExpr proto hook behavior and kept this PR's HashExpr hook migration, so HashExpr now goes through try_to_proto / try_from_proto rather than the central to_proto downcast branch.

Local validation passed:

• cargo fmt --check -- datafusion/proto/src/physical_plan/from_proto.rs datafusion/proto/src/physical_plan/to_proto.rs datafusion/physical-plan/src/joins/hash_join/partitioned_hash_eval.rs
• cargo test -p datafusion-physical-plan hash_expr_try --features proto --lib
• cargo check -p datafusion-proto
• git diff --check upstream/main...HEAD

GitHub checks have restarted on the new head.