feat(physical-expr): port Literal to try_to_proto / try_from_proto hooks

[koopatroopa787]

Which issue does this PR close?

Closes #22427

Rationale for this change

Literal serialization/deserialization lived in the central downcast chains in to_proto.rs and from_proto.rs. This PR moves it into self-contained try_to_proto / try_from_proto hooks on Literal itself, following the pattern established for NotExpr, NegativeExpr, IsNullExpr, and IsNotNullExpr.

What changes are included in this PR?

• datafusion/physical-expr/src/expressions/literal.rs
  • Added #[cfg(feature = "proto")] fn try_to_proto(...) inside impl PhysicalExpr for Literal
  • Added #[cfg(feature = "proto")] impl Literal { pub fn try_from_proto(...) }
  • Added proto_tests module with encode, null-literal, roundtrip, and reject-wrong-variant tests
• datafusion/proto/src/physical_plan/to_proto.rs
  • Removed the Literal downcast arm; removed Literal from import list
• datafusion/proto/src/physical_plan/from_proto.rs
  • Replaced the inline ExprType::Literal arm with Literal::try_from_proto(proto, &decode_ctx)?

Are there any user-facing changes?

No. Serialization behaviour is identical; only the code location changed.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Note

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Refactors the protobuf serialization/deserialization of Literal physical expressions to use the per-expression try_to_proto / try_from_proto hook pattern (instead of being centrally handled in to_proto.rs / from_proto.rs).

Changes:

• Removed the inline Literal arm from serialize_physical_expr_with_converter in to_proto.rs and dropped the unused Literal import.
• Routed ExprType::Literal decoding through Literal::try_from_proto in from_proto.rs.
• Added try_to_proto impl and try_from_proto constructor on Literal, plus unit tests under a proto_tests module.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
datafusion/proto/src/physical_plan/to_proto.rs	Removed the inline literal serialization branch and unused Literal import.
datafusion/proto/src/physical_plan/from_proto.rs	Delegates literal decoding to Literal::try_from_proto.
datafusion/physical-expr/src/expressions/literal.rs	Adds try_to_proto/try_from_proto methods and proto tests for Literal.

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[kosiew]

@koopatroopa787
Thanks for the fix. I took a look and did not find any blocking issues. I have one small suggestion that could strengthen the test coverage.

[kosiew]

Nice test. One thing that might make it a bit stronger is to verify the decoded payload as well, not just the proto variant.

Since the contract is that this round-trips as a null Int32 literal, could we also decode the literal payload and assert that it equals ScalarValue::Int32(None)? That would help ensure both the variant and the encoded value are preserved correctly.