refactor: Move MessageAggregator to spring-ai-model module

Improve separation of concerns by keeping model-related functionality in the model module
while maintaining client-specific functionality in the client module.

- Move MessageAggregator class from spring-ai-client-chat to spring-ai-model module
- Create new ChatClientMessageAggregator in spring-ai-client-chat module to handle client-specific aggregation
- Extract client-specific aggregation logic from MessageAggregator to ChatClientMessageAggregator
- Update references in advisor classes to use the new ChatClientMessageAggregator

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

spring-ai-client-chat/src/main/java/org/springframework/ai/chat/client/ChatClientMessageAggregator.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2023-2025 the original author or authors.
+ *
+ * Licensed 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
+ *
+ *      https://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.
+ */
+
+package org.springframework.ai.chat.client;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Consumer;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import reactor.core.publisher.Flux;
+
+import org.springframework.ai.chat.model.MessageAggregator;
+
+/**
+ * Helper that for streaming chat responses, aggregate the chat response messages into a
+ * single AssistantMessage. Job is performed in parallel to the chat response processing.
+ *
+ * @author Christian Tzolov
+ * @author Alexandros Pappas
+ * @author Thomas Vitale
+ * @since 1.0.0
+ */
+public class ChatClientMessageAggregator {
+
+	private static final Logger logger = LoggerFactory.getLogger(ChatClientMessageAggregator.class);
+
+	public Flux<ChatClientResponse> aggregateChatClientResponse(Flux<ChatClientResponse> chatClientResponses,
+			Consumer<ChatClientResponse> aggregationHandler) {
+
+		AtomicReference<Map<String, Object>> context = new AtomicReference<>(new HashMap<>());
+
+		return new MessageAggregator().aggregate(chatClientResponses.mapNotNull(chatClientResponse -> {
+			context.get().putAll(chatClientResponse.context());
+			return chatClientResponse.chatResponse();
+		}), aggregatedChatResponse -> {
+			ChatClientResponse aggregatedChatClientResponse = ChatClientResponse.builder()
+				.chatResponse(aggregatedChatResponse)
+				.context(context.get())
+				.build();
+			aggregationHandler.accept(aggregatedChatClientResponse);
+		}).map(chatResponse -> ChatClientResponse.builder().chatResponse(chatResponse).context(context.get()).build());
+	}
+
+}

spring-ai-client-chat/src/main/java/org/springframework/ai/chat/client/advisor/PromptChatMemoryAdvisor.java

@@ -28,6 +28,7 @@ import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Scheduler;
 import reactor.core.scheduler.Schedulers;
 
+import org.springframework.ai.chat.client.ChatClientMessageAggregator;
 import org.springframework.ai.chat.client.ChatClientRequest;
 import org.springframework.ai.chat.client.ChatClientResponse;
 import org.springframework.ai.chat.client.advisor.api.Advisor;
@@ -40,7 +41,6 @@ import org.springframework.ai.chat.messages.Message;
 import org.springframework.ai.chat.messages.MessageType;
 import org.springframework.ai.chat.messages.SystemMessage;
 import org.springframework.ai.chat.messages.UserMessage;
-import org.springframework.ai.chat.model.MessageAggregator;
 import org.springframework.ai.chat.prompt.PromptTemplate;
 
 /**
@@ -172,7 +172,7 @@ public class PromptChatMemoryAdvisor implements BaseChatMemoryAdvisor {
 			.publishOn(scheduler)
 			.map(request -> this.before(request, streamAdvisorChain))
 			.flatMapMany(streamAdvisorChain::nextStream)
-			.transform(flux -> new MessageAggregator().aggregateChatClientResponse(flux,
+			.transform(flux -> new ChatClientMessageAggregator().aggregateChatClientResponse(flux,
 					response -> this.after(response, streamAdvisorChain)));
 	}
 

spring-ai-client-chat/src/main/java/org/springframework/ai/chat/client/advisor/SimpleLoggerAdvisor.java

@@ -18,10 +18,11 @@ package org.springframework.ai.chat.client.advisor;
 
 import java.util.function.Function;
 
-import reactor.core.publisher.Flux;
-
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+import reactor.core.publisher.Flux;
+
+import org.springframework.ai.chat.client.ChatClientMessageAggregator;
 import org.springframework.ai.chat.client.ChatClientRequest;
 import org.springframework.ai.chat.client.ChatClientResponse;
 import org.springframework.ai.chat.client.advisor.api.CallAdvisor;
@@ -29,7 +30,6 @@ import org.springframework.ai.chat.client.advisor.api.CallAdvisorChain;
 import org.springframework.ai.chat.client.advisor.api.StreamAdvisor;
 import org.springframework.ai.chat.client.advisor.api.StreamAdvisorChain;
 import org.springframework.ai.chat.model.ChatResponse;
-import org.springframework.ai.chat.model.MessageAggregator;
 import org.springframework.ai.model.ModelOptionsUtils;
 import org.springframework.lang.Nullable;
 
@@ -85,7 +85,7 @@ public class SimpleLoggerAdvisor implements CallAdvisor, StreamAdvisor {
 
 		Flux<ChatClientResponse> chatClientResponses = streamAdvisorChain.nextStream(chatClientRequest);
 
-		return new MessageAggregator().aggregateChatClientResponse(chatClientResponses, this::logResponse);
+		return new ChatClientMessageAggregator().aggregateChatClientResponse(chatClientResponses, this::logResponse);
 	}
 
 	private void logRequest(ChatClientRequest request) {

spring-ai-model/src/main/java/org/springframework/ai/chat/model/MessageAggregator.java

@@ -24,7 +24,6 @@ import java.util.function.Consumer;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
-import org.springframework.ai.chat.client.ChatClientResponse;
 import reactor.core.publisher.Flux;
 
 import org.springframework.ai.chat.messages.AssistantMessage;
@@ -49,23 +48,6 @@ public class MessageAggregator {
 
 	private static final Logger logger = LoggerFactory.getLogger(MessageAggregator.class);
 
-	public Flux<ChatClientResponse> aggregateChatClientResponse(Flux<ChatClientResponse> chatClientResponses,
-			Consumer<ChatClientResponse> aggregationHandler) {
-
-		AtomicReference<Map<String, Object>> context = new AtomicReference<>(new HashMap<>());
-
-		return new MessageAggregator().aggregate(chatClientResponses.mapNotNull(chatClientResponse -> {
-			context.get().putAll(chatClientResponse.context());
-			return chatClientResponse.chatResponse();
-		}), aggregatedChatResponse -> {
-			ChatClientResponse aggregatedChatClientResponse = ChatClientResponse.builder()
-				.chatResponse(aggregatedChatResponse)
-				.context(context.get())
-				.build();
-			aggregationHandler.accept(aggregatedChatClientResponse);
-		}).map(chatResponse -> ChatClientResponse.builder().chatResponse(chatResponse).context(context.get()).build());
-	}
-
 	public Flux<ChatResponse> aggregate(Flux<ChatResponse> fluxChatResponse,
 			Consumer<ChatResponse> onAggregationComplete) {