refactor: redesign default model configuration with is_default field

Breaking Changes:
- Remove separate default_model config section
- Add is_default field to model configuration
- Simplify UI: set default model directly in model list
- Each provider can have one default model

Changes:
- Backend: Add is_default to LLMProviderModelConfig schema
- Frontend: Rewrite LLMSettingsSection with new design
- Add model helper functions for finding default model
- Update type definitions

Benefits:
- Single source of truth: no sync issues
- Better UX: set default directly in model list
- Simpler code: no complex derivation logic

Author: yhjun1026

packages/derisk-app/src/derisk_app/openapi/api_v1/helpers/__init__.py

@@ -0,0 +1,83 @@
+def find_default_model(config: AppConfig) -> Optional[Dict[str, Any]]:
+    """从配置中查找默认模型
+
+    Args:
+        config: 应用配置
+
+    Returns:
+        默认模型配置字典，包含 provider, model_name, temperature, max_new_tokens 等
+        如果没有找到返回 None
+    """
+    try:
+        agent_llm = getattr(config, "agent_llm", None)
+        if not agent_llm or not hasattr(agent_llm, "providers"):
+            return None
+
+        providers = agent_llm.providers or []
+
+        # 查找第一个标记为 is_default 的模型
+        for provider_config in providers:
+            if not hasattr(provider_config, "models"):
+                continue
+
+            models = provider_config.models or []
+            for model_config in models:
+                if getattr(model_config, "is_default", False):
+                    return {
+                        "provider": provider_config.provider,
+                        "model_name": model_config.name,
+                        "temperature": model_config.temperature or 0.7,
+                        "max_new_tokens": model_config.max_new_tokens or 4096,
+                        "is_multimodal": getattr(model_config, "is_multimodal", False),
+                        "api_base": provider_config.api_base,
+                        "api_key_ref": provider_config.api_key_ref,
+                    }
+
+        # 如果没有找到 is_default，返回第一个 provider 的第一个模型
+        if providers and hasattr(providers[0], "models") and providers[0].models:
+            first_provider = providers[0]
+            first_model = first_provider.models[0]
+            return {
+                "provider": first_provider.provider,
+                "model_name": first_model.name,
+                "temperature": first_model.temperature or 0.7,
+                "max_new_tokens": first_model.max_new_tokens or 4096,
+                "is_multimodal": getattr(first_model, "is_multimodal", False),
+                "api_base": first_provider.api_base,
+                "api_key_ref": first_provider.api_key_ref,
+            }
+
+        return None
+    except Exception as e:
+        logger.warning(f"Failed to find default model: {e}")
+        return None
+
+
+def get_all_models_from_config(config: AppConfig) -> List[str]:
+    """从配置中获取所有模型名称
+
+    Args:
+        config: 应用配置
+
+    Returns:
+        模型名称列表
+    """
+    models = []
+    try:
+        agent_llm = getattr(config, "agent_llm", None)
+        if not agent_llm or not hasattr(agent_llm, "providers"):
+            return models
+
+        providers = agent_llm.providers or []
+        for provider_config in providers:
+            if not hasattr(provider_config, "models"):
+                continue
+
+            for model_config in provider_config.models or []:
+                if hasattr(model_config, "name") and model_config.name:
+                    models.append(model_config.name)
+
+        return models
+    except Exception as e:
+        logger.warning(f"Failed to get all models from config: {e}")
+        return models

packages/derisk-app/src/derisk_app/openapi/api_v1/helpers/model_helper.py

@@ -139,12 +139,13 @@ class OAuth2Config(BaseModel):
 
 
 class LLMProviderModelConfig(BaseModel):
-    """LLM Provider 中的模型配置"""
+    """模型配置（provider下的模型）"""
 
-    name: str = "gpt-4"
-    temperature: float = 0.7
-    max_new_tokens: int = 4096
-    is_multimodal: bool = False
+    name: str = Field(..., description="模型名称，如 gpt-4o, deepseek-chat")
+    temperature: float = Field(0.7, description="模型温度参数")
+    max_new_tokens: int = Field(4096, description="最大生成token数")
+    is_multimodal: bool = Field(False, description="是否支持多模态（图片输入）")
+    is_default: bool = Field(False, description="是否为该provider下的默认模型")
 
 
 class LLMProviderConfig(BaseModel):