Use stream json api in protobuf v31+

[chenBright]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

protocolbuffers/protobuf@cff2bb5#diff-a7f751e303f1778830da9721fb6dc4f02aa9208a88c7374aa1556682d92d8c63
Protobuf v31+ supports streaming APIs for converting between JSON and messages.

Streaming API wrappers using bRPC are no longer needed.

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull request overview

This PR updates bRPC’s protobuf↔ProtoJSON conversion path to use protobuf’s native streaming JSON APIs when building against protobuf v31+ (per GOOGLE_PROTOBUF_VERSION gating), reducing reliance on bRPC’s legacy wrapper approach while retaining the existing fallback for older protobuf versions.

Changes:

• Switch ProtoMessageToProtoJson(...) to google::protobuf::json::MessageToJsonStream(...) for GOOGLE_PROTOBUF_VERSION >= 6031000.
• Switch ProtoJsonToProtoMessage(...) to google::protobuf::json::JsonStreamToMessage(...) for GOOGLE_PROTOBUF_VERSION >= 6031000.
• Mark brpc_load_balancer_unittest.cpp as size=large in Bazel to mitigate CI timeouts.

Reviewed changes

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

File	Description
test/BUILD.bazel	Increases Bazel timeout class for brpc_load_balancer_unittest.cpp by setting it to large.
src/json2pb/pb_to_json.cpp	Uses protobuf’s streaming Message→JSON API when available; keeps the existing BinaryToJsonStream fallback otherwise.
src/json2pb/json_to_pb.cpp	Uses protobuf’s streaming JSON→Message API when available; keeps the existing JsonToBinaryStream+Parse fallback otherwise.

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