refactor: Migrate ScalarSubqueryExpr to self-serialization proto pattern

[mattp5657]

Move ScalarSubqueryExpr proto encode/decode into the expression itself via the try_to_proto / try_from_proto hooks, removing the centralized downcast branch in to_proto.rs and the inline construction in from_proto.rs. Because the ScalarSubqueryResults container is runtime-only shared state and not part of the wire format, from_proto.rs still fetches it from the decode context and threads it into try_from_proto.

Follows the pattern established in #21929.

Which issue does this PR close?

• Closes #22433.

Rationale for this change

datafusion-proto serializes ScalarSubqueryExpr by downcasting in to_proto.rs and rebuilding it inline in from_proto.rs, which forces its internal fields to be exposed as public accessors just for the proto crate.

PR #21929 introduced a self-serialization pattern (try_to_proto / try_from_proto) where each expression owns its own wire format and the central dispatch just delegates. Other expressions like InListExpr and DynamicFilterPhysicalExpr already use it. This change moves ScalarSubqueryExpr to the same pattern, removing its bespoke proto branches and the proto-only public accessors.

One wrinkle: ScalarSubqueryExpr's ScalarSubqueryResults is runtime-only shared state, not part of the wire format, so try_from_proto takes it as an extra argument that from_proto.rs threads in from the decode context.

What changes are included in this PR?

• try_to_proto in impl PhysicalExpr for ScalarSubqueryExpr (not inherent), #[cfg(feature = "proto")]
• ScalarSubqueryExpr::try_from_proto wired into from_proto.rs
• central to_proto.rs + from_proto.rs arms deleted in this PR
• direct hook test incl. bad-input case; test module at end of file
• roundtrip tests pass; fmt + clippy -D warnings clean; PR template filled in

Are these changes tested?

• Unit tests (scalar_subquery.rs, #[cfg(all(test, feature = "proto"))] module at end of file) exercise the hooks directly:
  • round_trips_through_proto — encode via try_to_proto, decode via try_from_proto, asserting data_type/nullable/index and shared-results identity survive.
  • rejects_non_scalar_subquery_node — wrong ExprType variant errors cleanly.
  • rejects_missing_data_type — missing required field errors cleanly.
• Round-trip integration tests (datafusion-proto, --all-features) cover the full plan path: roundtrip_scalar_subquery_exec, roundtrip_nested_scalar_subquery_exec_scopes_results, and roundtrip_scalar_subquery_exec_with_default_converter_executes.

All pass, and cargo fmt --all + cargo clippy --all-features --all-targets -- -D warnings are clean.

Are there any user-facing changes?

No