THRIFT-6119: Preserve Ruby multiplexed protocol service names

Client: rb

Co-Authored-By: OpenAI Codex (GPT-5.6) <codex@openai.com>

Author: kpumuk

lib/rb/lib/thrift/protocol/multiplexed_protocol.rb

@@ -39,7 +39,7 @@ def write_message_begin(name, type, seqid)
     end
 
     def to_s
-      "multiplexed(#{@service_name=@protocol.to_s})"
+      "multiplexed(#{@protocol})"
     end
   end
 end

lib/rb/spec/multiplexed_protocol_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements. See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership. The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License. You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied. See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+require 'spec_helper'
+
+describe Thrift::MultiplexedProtocol do
+  it 'keeps its configured service name when formatted for diagnostics' do
+    transport = Thrift::MemoryBufferTransport.new
+    protocol = Thrift::MultiplexedProtocol.new(Thrift::BinaryProtocol.new(transport), 'Calculator')
+
+    expect(protocol.to_s).to eq('multiplexed(binary(memory))')
+    expect(protocol.to_s).to eq('multiplexed(binary(memory))')
+
+    protocol.write_message_begin('add', Thrift::MessageTypes::CALL, 7)
+
+    name, type, seqid = Thrift::BinaryProtocol.new(transport).read_message_begin
+    expect([name, type, seqid]).to eq(['Calculator:add', Thrift::MessageTypes::CALL, 7])
+  end
+end