Use stream json api in protobuf 31 and later versions

[chenBright]

What problem does this PR solve?

Issue Number:

Problem Summary:

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

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• Please make sure your changes are compilable.
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[Copilot]

Pull request overview

This PR updates the json2pb ProtoJSON conversion paths to use protobuf’s newer streaming JSON APIs when building against protobuf 6.31+, reducing intermediate buffering and leveraging the upstream JSON implementation directly.

Changes:

• Use google::protobuf::json::MessageToJsonStream for ProtoMessage → ProtoJSON when GOOGLE_PROTOBUF_VERSION >= 6031000.
• Use google::protobuf::json::JsonStreamToMessage for ProtoJSON → ProtoMessage when GOOGLE_PROTOBUF_VERSION >= 6031000.
• Keep the existing google::protobuf::util::{BinaryToJsonStream, JsonToBinaryStream} fallback for older protobuf versions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/json2pb/pb_to_json.cpp	Switch ProtoJSON serialization to protobuf’s streaming JSON API for protobuf 6.31+ with a fallback for older versions.
src/json2pb/json_to_pb.cpp	Switch ProtoJSON parsing to protobuf’s streaming JSON API for protobuf 6.31+ with a fallback for older versions.

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