Port CaseExpr to use try_to_proto / try_from_proto #22421

Sub-issue of #22418. Port CaseExpr to own its protobuf (de)serialization via the try_to_proto / try_from_proto hooks added in #21929, removing it from the central downcast chain.

Reference: copy the Column migration in #21929. Mind the asymmetry — try_to_proto is a trait override, try_from_proto is an inherent fn (see steps).

Where CaseExpr lives

datafusion/physical-expr/src/expressions/case.rs (crate datafusion-physical-expr).

Steps

1. Add try_to_proto inside the impl PhysicalExpr for CaseExpr block — it overrides a trait method, so it must live there, feature-gated #[cfg(feature = "proto")].
   ⚠️ Do not put it in an inherent impl CaseExpr block. The serializer calls it through &dyn PhysicalExpr, so an inherent method is silently never reached and serialization keeps using the old chain. This is the most common review bounce on these PRs.
2. Add CaseExpr::try_from_proto(node, ctx) as an inherent associated fn (#[cfg(feature = "proto")] impl CaseExpr) and wire it into the matching ExprType::… arm in datafusion/proto/src/physical_plan/from_proto.rs.
3. In this same PR, delete the CaseExpr arm from datafusion/proto/src/physical_plan/to_proto.rs and the old ExprType::… decode arm in from_proto.rs. This is not cleanup-for-later — it is the only thing that proves your hook is actually reached. If the roundtrip tests still pass after deleting the arm, it works; if you leave the arm, the tests pass through dead code.
4. Keep the wire format byte-for-byte identical. expr_id is None here (only DynamicFilterPhysicalExpr sets it).
5. Keep use datafusion_proto_models::protobuf; inside the method bodies (matches the Column reference).

Tests

• Existing roundtrip_physical_plan / roundtrip_physical_expr must still pass: cargo test -p datafusion-proto --test proto_integration.
• Add a direct hook test in datafusion/physical-expr/src/expressions/case.rs covering both directions and a bad-input case (wrong expr_type variant / missing child → clean error, not panic). Reviewers have asked for this on every reviewed PR.
• Put #[cfg(test)] mod tests at the end of the file or clippy -D warnings fails (items-after-test-module).

Before you push

• Fill in every section of the PR template.
• cargo fmt --all and cargo clippy --all-targets --all-features -- -D warnings must pass.

Checklist

• try_to_proto in impl PhysicalExpr for CaseExpr (not inherent), #[cfg(feature="proto")]
• CaseExpr::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