feat: 优化 Swagger UI docs 默认参数和字段描述

main.py 变更:
- ChatMessage/ChatCompletionRequest 使用 Pydantic Field 添加 description 和 example
- model 默认 example 设为 'qwen-plus'，提供 ['', 'qwen-flash', 'qwen-plus', 'qwen-max'] 选项
- messages 默认 example 设为 [{role: 'user', content: '你好，介绍一下你自己'}]
- 所有字段添加中文描述和取值范围约束 (ge/le)
- 修复 Swagger UI 点击 Try it out 后直接 Execute 报 'Unknown model: string' 的问题

效果:
- /docs 页面现在显示合理的默认值，一键即可测试
- 每个字段都有清晰的中文说明和取值范围提示

main.py

@@ -58,23 +58,33 @@ _load_history()
 
 
 # ── OpenAI 兼容请求/响应模型 ────────────────────────────────
+from pydantic import BaseModel, Field
 class ChatMessage(BaseModel):
-    role: str
+    role: str = Field(..., description="角色：system, user, assistant", example="user")
-    content: Optional[str] = None
+    content: Optional[str] = Field(None, description="消息内容", example="你好，介绍一下你自己")
-    name: Optional[str] = None
+    name: Optional[str] = Field(None, description="可选的名称")
 
 class ChatCompletionRequest(BaseModel):
-    model: Optional[str] = None
+    model: Optional[str] = Field(
-    messages: List[ChatMessage]
+        None, 
-    temperature: Optional[float] = 0.7
+        description="模型名称（留空时自动使用 NVIDIA 分类器智能路由）",
-    max_tokens: Optional[int] = None
+        example="qwen-plus",
-    stream: Optional[bool] = False
+        json_schema_extra={"examples": ["", "qwen-flash", "qwen-plus", "qwen-max"]}
-    top_p: Optional[float] = 1.0
+    )
-    n: Optional[int] = 1
+    messages: List[ChatMessage] = Field(
-    stop: Optional[Any] = None
+        ..., 
-    presence_penalty: Optional[float] = 0.0
+        description="对话消息列表",
-    frequency_penalty: Optional[float] = 0.0
+        example=[{"role": "user", "content": "你好，介绍一下你自己"}]
-    user: Optional[str] = None
+    )
+    temperature: Optional[float] = Field(0.7, ge=0, le=2, description="随机性 (0-2)")
+    max_tokens: Optional[int] = Field(None, ge=1, description="最大生成 token 数")
+    stream: Optional[bool] = Field(False, description="是否使用流式输出")
+    top_p: Optional[float] = Field(1.0, ge=0, le=1, description="核采样参数")
+    n: Optional[int] = Field(1, ge=1, le=10, description="生成回复数量")
+    stop: Optional[Any] = Field(None, description="停止词")
+    presence_penalty: Optional[float] = Field(0.0, ge=-2, le=2, description="存在惩罚")
+    frequency_penalty: Optional[float] = Field(0.0, ge=-2, le=2, description="频率惩罚")
+    user: Optional[str] = Field(None, description="用户标识")
 
 
 # ── FastAPI App ──────────────────────────────────────────────