feat(api): Add image multimodal support for LLMNode (#17372)

Enhance `LLMNode` with multimodal capability, introducing support for
image outputs.

This implementation extracts base64-encoded images from LLM responses,
saves them to the storage service, and records the file metadata in the
`ToolFile` table. In conversations, these images are rendered as
markdown-based inline images.
Additionally, the images are included in the LLMNode's output as
file variables, enabling subsequent nodes in the workflow to utilize them.

To integrate file outputs into workflows, adjustments to the frontend code
are necessary.

For multimodal output functionality, updates to related model configurations
are required. Currently, this capability has been applied exclusively to
Google's Gemini models.

Close #15814.

Signed-off-by: -LAN- <laipz8200@outlook.com>
Co-authored-by: -LAN- <laipz8200@outlook.com>

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

@@ -2,7 +2,7 @@ import logging
 import time
 import uuid
 from collections.abc import Generator, Sequence
-from typing import Optional, Union, cast
+from typing import Optional, Union
 
 from pydantic import ConfigDict
 
@@ -13,14 +13,15 @@ from core.model_runtime.entities.llm_entities import LLMResult, LLMResultChunk,
 from core.model_runtime.entities.message_entities import (
     AssistantPromptMessage,
     PromptMessage,
+    PromptMessageContentUnionTypes,
     PromptMessageTool,
+    TextPromptMessageContent,
 )
 from core.model_runtime.entities.model_entities import (
     ModelType,
     PriceType,
 )
 from core.model_runtime.model_providers.__base.ai_model import AIModel
-from core.model_runtime.utils.helper import convert_llm_result_chunk_to_str
 from core.plugin.impl.model import PluginModelClient
 
 logger = logging.getLogger(__name__)
@@ -238,7 +239,7 @@ class LargeLanguageModel(AIModel):
     def _invoke_result_generator(
         self,
         model: str,
-        result: Generator,
+        result: Generator[LLMResultChunk, None, None],
         credentials: dict,
         prompt_messages: Sequence[PromptMessage],
         model_parameters: dict,
@@ -255,11 +256,21 @@ class LargeLanguageModel(AIModel):
         :return: result generator
         """
         callbacks = callbacks or []
-        assistant_message = AssistantPromptMessage(content="")
+        message_content: list[PromptMessageContentUnionTypes] = []
         usage = None
         system_fingerprint = None
         real_model = model
 
+        def _update_message_content(content: str | list[PromptMessageContentUnionTypes] | None):
+            if not content:
+                return
+            if isinstance(content, list):
+                message_content.extend(content)
+                return
+            if isinstance(content, str):
+                message_content.append(TextPromptMessageContent(data=content))
+                return
+
         try:
             for chunk in result:
                 # Following https://github.com/langgenius/dify/issues/17799,
@@ -281,9 +292,8 @@ class LargeLanguageModel(AIModel):
                     callbacks=callbacks,
                 )
 
-                text = convert_llm_result_chunk_to_str(chunk.delta.message.content)
-                current_content = cast(str, assistant_message.content)
-                assistant_message.content = current_content + text
+                _update_message_content(chunk.delta.message.content)
+
                 real_model = chunk.model
                 if chunk.delta.usage:
                     usage = chunk.delta.usage
@@ -293,6 +303,7 @@ class LargeLanguageModel(AIModel):
         except Exception as e:
             raise self._transform_invoke_error(e)
 
+        assistant_message = AssistantPromptMessage(content=message_content)
         self._trigger_after_invoke_callbacks(
             model=model,
             result=LLMResult(