fix: JSON mode with an image doesn't work for Gemini (#5169)

api/core/model_runtime/model_providers/__base/large_language_model.py

@@ -14,6 +14,7 @@ from core.model_runtime.entities.llm_entities import LLMMode, LLMResult, LLMResu
 from core.model_runtime.entities.message_entities import (
     AssistantPromptMessage,
     PromptMessage,
+    PromptMessageContentType,
     PromptMessageTool,
     SystemPromptMessage,
     UserPromptMessage,
@@ -205,8 +206,14 @@ if you are not sure about the structure.
             ))
 
         if len(prompt_messages) > 0 and isinstance(prompt_messages[-1], UserPromptMessage):
-            # add ```JSON\n to the last message
-            prompt_messages[-1].content += f"\n```{code_block}\n"
+            # add ```JSON\n to the last text message
+            if isinstance(prompt_messages[-1].content, str):
+                prompt_messages[-1].content += f"\n```{code_block}\n"
+            elif isinstance(prompt_messages[-1].content, list):
+                for i in range(len(prompt_messages[-1].content) - 1, -1, -1):
+                    if prompt_messages[-1].content[i].type == PromptMessageContentType.TEXT:
+                        prompt_messages[-1].content[i].data += f"\n```{code_block}\n"
+                        break
         else:
             # append a user message
             prompt_messages.append(UserPromptMessage(