Add VertexAI Gemini system-instruction support

 Resolves #906

Author: tzolov

models/spring-ai-vertex-ai-gemini/src/main/java/org/springframework/ai/vertexai/gemini/VertexAiGeminiChatModel.java

@@ -28,6 +28,7 @@
 import com.google.cloud.vertexai.api.Part;
 import com.google.cloud.vertexai.api.Schema;
 import com.google.cloud.vertexai.api.Tool;
+import com.google.cloud.vertexai.generativeai.ContentMaker;
 import com.google.cloud.vertexai.generativeai.GenerativeModel;
 import com.google.cloud.vertexai.generativeai.PartMaker;
 import com.google.cloud.vertexai.generativeai.ResponseStream;
@@ -258,6 +259,16 @@ private GeminiRequest createGeminiRequest(Prompt prompt) {
 
 		GenerativeModel generativeModel = generativeModelBuilder.build();
 
+		String systemContext = prompt.getInstructions()
+			.stream()
+			.filter(m -> m.getMessageType() == MessageType.SYSTEM)
+			.map(m -> m.getContent())
+			.collect(Collectors.joining(System.lineSeparator()));
+
+		if (StringUtils.hasText(systemContext)) {
+			generativeModel.withSystemInstruction(ContentMaker.fromString(systemContext));
+		}
+
 		return new GeminiRequest(toGeminiContent(prompt), generativeModel);
 	}
 
@@ -289,18 +300,12 @@ private GenerationConfig toGenerationConfig(VertexAiGeminiChatOptions options) {
 
 	private List<Content> toGeminiContent(Prompt prompt) {
 
-		String systemContext = prompt.getInstructions()
-			.stream()
-			.filter(m -> m.getMessageType() == MessageType.SYSTEM)
-			.map(m -> m.getContent())
-			.collect(Collectors.joining(System.lineSeparator()));
-
 		List<Content> contents = prompt.getInstructions()
 			.stream()
 			.filter(m -> m.getMessageType() == MessageType.USER || m.getMessageType() == MessageType.ASSISTANT)
 			.map(message -> Content.newBuilder()
 				.setRole(toGeminiMessageType(message.getMessageType()).getValue())
-				.addAllParts(messageToGeminiParts(message, systemContext))
+				.addAllParts(messageToGeminiParts(message))
 				.build())
 			.toList();
 
@@ -321,14 +326,11 @@ private static GeminiMessageType toGeminiMessageType(@NonNull MessageType type)
 		}
 	}
 
-	static List<Part> messageToGeminiParts(Message message, String systemContext) {
+	static List<Part> messageToGeminiParts(Message message) {
 
 		if (message instanceof UserMessage userMessage) {
 
 			String messageTextContent = (userMessage.getContent() == null) ? "null" : userMessage.getContent();
-			if (StringUtils.hasText(systemContext)) {
-				messageTextContent = systemContext + "\n\n" + messageTextContent;
-			}
 			Part textPart = Part.newBuilder().setText(messageTextContent).build();
 
 			List<Part> parts = new ArrayList<>(List.of(textPart));

models/spring-ai-vertex-ai-gemini/src/test/java/org/springframework/ai/vertexai/gemini/function/VertexAiGeminiChatModelFunctionCallingIT.java

@@ -28,7 +28,6 @@
 import org.slf4j.LoggerFactory;
 import org.springframework.ai.chat.messages.AssistantMessage;
 import org.springframework.ai.chat.messages.Message;
-import org.springframework.ai.chat.messages.SystemMessage;
 import org.springframework.ai.chat.messages.UserMessage;
 import org.springframework.ai.chat.model.ChatResponse;
 import org.springframework.ai.chat.model.Generation;