feat: 增强Gemini API tools参数处理

- 修改GeminiRequest模型，使tools字段支持单个工具对象或工具对象列表
- 在gemini_chat_service中添加类型转换逻辑，确保tools始终以列表形式处理
- 提高API的灵活性和兼容性

app/domain/gemini_models.py

@@ -1,4 +1,4 @@
-from typing import List, Optional, Dict, Any, Literal
+from typing import List, Optional, Dict, Any, Literal, Union
 from pydantic import BaseModel
 
 
@@ -34,7 +34,7 @@ class GeminiContent(BaseModel):
 
 class GeminiRequest(BaseModel):
     contents: List[GeminiContent] = []
-    tools: Optional[List[Dict[str, Any]]] = []
+    tools: Optional[Union[List[Dict[str, Any]], Dict[str, Any]]] = []
     safetySettings: Optional[List[SafetySetting]] = None
     generationConfig: Optional[GenerationConfig] = None
     systemInstruction: Optional[SystemInstruction] = None