Fix continuedev#4723/continuedev#4774: Add error handling for system prompt

+ Ensure that Claude models on VertexAI send
  correct prompts, even if the systemMessage
  is undefined.

Signed-off-by: "Michael Elder" @mdelder

Author: mdelder

core/llm/llms/Gemini.ts

@@ -4,6 +4,7 @@ import {
   CompletionOptions,
   LLMOptions,
   MessagePart,
+  TextMessagePart,
   ToolCallDelta,
 } from "../../index.js";
 import { findLast } from "../../util/findLast.js";
@@ -69,21 +70,71 @@ class Gemini extends BaseLLM {
     }
   }
 
-  public removeSystemMessage(messages: ChatMessage[]): ChatMessage[] {
-    // should be public for use within VertexAI
-    const msgs = [...messages];
-
-    if (msgs[0]?.role === "system") {
-      const sysMsg = msgs.shift()?.content;
-      // @ts-ignore
-      if (msgs[0]?.role === "user") {
-        // @ts-ignore
-        msgs[0].content = `System message - follow these instructions in every response: ${sysMsg}\n\n---\n\n${msgs[0].content}`;
+/**
+ * Removes the system message and merges it with the next user message if present.
+ * @param messages Array of chat messages
+ * @returns Modified array with system message merged into user message if applicable
+ */
+public removeSystemMessage(messages: ChatMessage[]): ChatMessage[] {
+  // If no messages or first message isn't system, return copy of original messages
+  if (messages.length === 0 || messages[0]?.role !== "system") {
+    return [...messages];
+  }
+
+  // Extract system message
+  const systemMessage : ChatMessage = messages[0];
+  
+  // Extract system content based on its type
+  let systemContent = "";
+  if (typeof systemMessage.content === "string") {
+    systemContent = systemMessage.content;
+  } else if (Array.isArray(systemMessage.content)) {
+    const contentArray : Array<MessagePart> = systemMessage.content as Array<MessagePart>;
+    const concatenatedText = contentArray
+      .filter(part => part.type === "text")
+      .map(part => part.text)
+      .join(" ");
+    systemContent = concatenatedText ? concatenatedText : "";
+  } else if (systemMessage.content && typeof systemMessage.content === "object") {
+    const typedContent = systemMessage.content as TextMessagePart;
+    systemContent = typedContent?.text || "";
+  }
+  
+  // Create new array without the system message
+  const remainingMessages : ChatMessage[] = messages.slice(1);
+  
+  // Check if there's a user message to merge with
+  if (remainingMessages.length > 0 && remainingMessages[0].role === "user") {
+    const userMessage : ChatMessage = remainingMessages[0];
+    const prefix = `System message - follow these instructions in every response: ${systemContent}\n\n---\n\n`;
+    
+    // Merge based on user content type
+    if (typeof userMessage.content === "string") {
+      userMessage.content = prefix + userMessage.content;
+    } else if (Array.isArray(userMessage.content)) {
+      const contentArray : Array<MessagePart> = userMessage.content as Array<MessagePart>;
+      const textPart = contentArray.find(part => part.type === "text") as TextMessagePart | undefined;
+      
+      if (textPart) {
+        textPart.text = prefix + textPart.text;
+      } else {
+        userMessage.content.push({
+          type: "text",
+          text: prefix
+        } as TextMessagePart);
       }
+    } else if (userMessage.content && typeof userMessage.content === "object") {
+      const typedContent = userMessage.content as TextMessagePart;
+      userMessage.content = [{
+        type: "text",
+        text: prefix + (typedContent.text || "")
+      } as TextMessagePart];
     }
-
-    return msgs;
   }
+  
+  return remainingMessages;
+}
+
 
   protected async *_streamChat(
     messages: ChatMessage[],