fix: return UnsupportedOperationError (-32004) when streaming is not supported

[ez-lbz]

What changed

1. Streaming-not-supported now returns UnsupportedOperationError (-32004) instead of InvalidRequestError (-32600)

Problem: When the agent card does not advertise streaming() capability, the JSON-RPC, gRPC, and REST handlers rejected streaming calls with InvalidRequestError (code -32600, "invalid request"), while other SDKs return UnsupportedOperationError (code -32004, "operation not supported"). -32600 is the wrong semantic for a valid-but-unavailable operation and diverges from the other SDKs.

Fix (transport/jsonrpc/src/main/java/org/a2aproject/sdk/transport/jsonrpc/handler/JSONRPCHandler.java):

• onMessageSendStream and onSubscribeToTask now emit new UnsupportedOperationError(null, "Streaming is not supported by the agent", null) (message preserved, code now -32004). Removed the now-unused InvalidRequestError import.

Fix (transport/grpc/src/main/java/org/a2aproject/sdk/transport/grpc/handler/GrpcHandler.java):

• sendStreamingMessage and subscribeToTask now call handleError(responseObserver, new UnsupportedOperationError()); the gRPC status therefore maps from INVALID_ARGUMENT to UNIMPLEMENTED. Updated the method javadoc accordingly.

Fix (transport/rest/src/main/java/org/a2aproject/sdk/transport/rest/handler/RestHandler.java):

• sendStreamingMessage and subscribeToTask now build the error response with new UnsupportedOperationError(null, "Streaming is not supported by the agent", null). HTTP status stays 400; the error reason changes from INVALID_REQUEST to UNSUPPORTED_OPERATION (via A2AErrorCodes.UNSUPPORTED_OPERATION).

Fix (tests updated in lockstep):

• JSONRPCHandlerTest (testStreamingNotSupportedErrorOnSendMessageStream, testStreamingNotSupportedErrorOnSubscribeToTask): assert UnsupportedOperationError instead of InvalidRequestError.
• GrpcHandlerTest (testStreamingNotSupportedError, testStreamingNotSupportedErrorOnSubscribeToTask): assert gRPC Status.Code.UNIMPLEMENTED instead of INVALID_ARGUMENT.
• RestHandlerTest (testSendStreamingMessageNotSupported): assert reason UNSUPPORTED_OPERATION instead of INVALID_REQUEST.

Behavior change: the JSON-RPC error code for streaming-not-supported changes from -32600 to -32004; the gRPC status changes from INVALID_ARGUMENT to UNIMPLEMENTED; the REST error reason changes from INVALID_REQUEST to UNSUPPORTED_OPERATION (HTTP status remains 400). The error message is unchanged.

Note: the compat-0.3 module (legacy 0.3-spec compatibility shim) has its own copies of the handlers with InvalidRequestError for this path; it was intentionally left unchanged as it is outside the scope of the current transports.

Testing

• mvn -pl transport/jsonrpc,transport/grpc,transport/rest test — 124 tests run, 0 failures, 1 skipped (BUILD SUCCESS): JSON-RPC 48 (1 skipped), gRPC 40, REST 36.