添加provider工厂和Ollama支持

- 实现ProviderFactory用于构建不同类型的provider适配器
- 添加EnvironmentCredentialResolver用于解析环境变量中的凭证
- 实现OllamaProvider支持本地模型调用
- 实现OpenAICompatibleProvider支持OpenAI兼容接口
- 在AgentRuntime中添加对ProviderError的处理
- 更新Message结构体添加tool_calls字段
- 实现provider配置的增删改查API端点
- 添加provider注册表的replace方法
- 添加HTTP基础类和工具参数解码功能
- 更新依赖添加httpx库
- 添加相关单元测试验证provider适配器功能
```

backend/app/providers/factory.py

@@ -0,0 +1,48 @@
+from app.contracts import ModelCapability, ProviderConfig, ProviderType
+from app.providers.base import ModelProvider
+from app.providers.credentials import CredentialResolver
+from app.providers.ollama import OllamaProvider
+from app.providers.openai_compatible import OpenAICompatibleProvider
+
+
+class UnsupportedProviderError(ValueError):
+    pass
+
+
+class ProviderFactory:
+    def __init__(self, credentials: CredentialResolver) -> None:
+        self.credentials = credentials
+
+    def build(self, config: ProviderConfig) -> ModelProvider:
+        if config.provider_type in {
+            ProviderType.openai_chat,
+            ProviderType.openai_compatible,
+        }:
+            return OpenAICompatibleProvider(
+                base_url=config.base_url or "https://api.openai.com/v1",
+                credential_id=config.credential_id,
+                credentials=self.credentials,
+            )
+        if config.provider_type == ProviderType.ollama:
+            return OllamaProvider(config.base_url or "http://127.0.0.1:11434")
+        raise UnsupportedProviderError(config.provider_type.value)
+
+    @staticmethod
+    def capabilities(provider_type: ProviderType) -> list[ModelCapability]:
+        if provider_type in {
+            ProviderType.openai_chat,
+            ProviderType.openai_compatible,
+        }:
+            return [
+                ModelCapability.chat,
+                ModelCapability.tool_calling,
+                ModelCapability.streaming,
+                ModelCapability.structured_output,
+            ]
+        if provider_type == ProviderType.ollama:
+            return [
+                ModelCapability.chat,
+                ModelCapability.tool_calling,
+                ModelCapability.streaming,
+            ]
+        return []