fix: support leader lookup with Oxia 0.16.x

[mattisonchao]

Motivation

The Java client supports shard-leader redirects from the google.rpc.ErrorInfo metadata used by Oxia 0.17.0 and newer. Oxia 0.16.x instead returns custom gRPC status code 106 with an io.oxia.proto.v1.LeaderHint protobuf detail.

gRPC Java normalizes code 106 to UNKNOWN and rejects the rich status because the normalized code no longer matches the status-details trailer. As a result, the client cannot see the leader hint and can keep retrying a stale cached leader after a failover.

Modifications

• Add the legacy LeaderHint message with the same wire fields used by Oxia 0.16.x.
• Introduce an ordered status-decoder strategy for the legacy 0.16.x and standard 0.17.x wire formats.
• Decode custom code 106 and LeaderHint directly into an OxiaStatusException in LegacyLeaderHintStatusDecoder.
• Keep validated gRPC status, ErrorInfo, standard-code, and message fallback handling in StandardGrpcStatusDecoder.
• Return UNKNOWN when a decoder does not recognize the error so OxiaStatusException can continue to the next strategy without an intermediate result or Optional.

Testing

• ./gradlew :client:spotlessCheck
• ./gradlew :client:spotbugsMain
• ./gradlew :client:test --tests "*Test"
• ./gradlew :client:test --tests io.oxia.client.grpc.OxiaStatusExceptionTest --tests io.oxia.client.grpc.GrpcRpcProviderTest