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/ollama.py

@@ -0,0 +1,117 @@
+from uuid import uuid4
+
+import httpx
+
+from app.contracts import ModelCapability, ModelInfo, ModelRequest
+from app.providers.base import ProviderError, ProviderToolCall, ProviderTurn
+from app.providers.http_base import TurnStreamingMixin, decode_tool_arguments
+
+
+class OllamaProvider(TurnStreamingMixin):
+    def __init__(
+        self,
+        base_url: str = "http://127.0.0.1:11434",
+        timeout_seconds: float = 120,
+        transport: httpx.AsyncBaseTransport | None = None,
+    ) -> None:
+        self.base_url = base_url.rstrip("/")
+        self.timeout_seconds = timeout_seconds
+        self.transport = transport
+
+    async def complete(self, request: ModelRequest) -> ProviderTurn:
+        messages = []
+        if request.system:
+            messages.append({"role": "system", "content": request.system})
+        for message in request.messages:
+            item: dict[str, object] = {
+                "role": message.role.value,
+                "content": message.content,
+            }
+            if message.tool_calls:
+                item["tool_calls"] = [
+                    {
+                        "function": {
+                            "name": call.name,
+                            "arguments": call.arguments,
+                        }
+                    }
+                    for call in message.tool_calls
+                ]
+            messages.append(item)
+        payload: dict[str, object] = {
+            "model": request.model,
+            "messages": messages,
+            "stream": False,
+        }
+        if request.tools:
+            payload["tools"] = [
+                {
+                    "type": "function",
+                    "function": {
+                        "name": tool.name,
+                        "description": tool.description,
+                        "parameters": tool.parameters,
+                    },
+                }
+                for tool in request.tools
+            ]
+        data = await self._request("POST", "/api/chat", json=payload)
+        message = data.get("message") or {}
+        tool_calls = []
+        for raw_call in message.get("tool_calls") or []:
+            function = raw_call.get("function") or {}
+            tool_calls.append(
+                ProviderToolCall(
+                    tool_call_id=raw_call.get("id") or f"call_{uuid4().hex}",
+                    name=function.get("name") or "",
+                    arguments=decode_tool_arguments(function.get("arguments", {})),
+                )
+            )
+        return ProviderTurn(
+            text=message.get("content") or None,
+            tool_calls=tool_calls,
+            input_tokens=int(data.get("prompt_eval_count") or 0),
+            output_tokens=int(data.get("eval_count") or 0),
+        )
+
+    async def list_models(self) -> list[ModelInfo]:
+        data = await self._request("GET", "/api/tags")
+        return [
+            ModelInfo(
+                model=item["name"],
+                display_name=item.get("name", ""),
+                capabilities=[ModelCapability.chat, ModelCapability.streaming],
+            )
+            for item in data.get("models", [])
+            if isinstance(item, dict) and item.get("name")
+        ]
+
+    async def test_connection(self, model: str | None = None) -> tuple[bool, str]:
+        try:
+            models = await self.list_models()
+        except ProviderError as exc:
+            return False, exc.message
+        if model and model not in {item.model for item in models}:
+            return False, f"Model is not installed: {model}"
+        return True, f"Connected; discovered {len(models)} local model(s)."
+
+    async def _request(self, method: str, path: str, **kwargs) -> dict:
+        try:
+            async with httpx.AsyncClient(
+                timeout=self.timeout_seconds, transport=self.transport
+            ) as client:
+                response = await client.request(method, f"{self.base_url}{path}", **kwargs)
+                response.raise_for_status()
+                data = response.json()
+        except httpx.TimeoutException as exc:
+            raise ProviderError("PROVIDER_TIMEOUT", "Ollama request timed out.") from exc
+        except httpx.HTTPStatusError as exc:
+            raise ProviderError(
+                "MODEL_NOT_FOUND" if exc.response.status_code == 404 else "PROVIDER_UNAVAILABLE",
+                f"Ollama returned HTTP {exc.response.status_code}.",
+            ) from exc
+        except (httpx.HTTPError, ValueError) as exc:
+            raise ProviderError("PROVIDER_UNAVAILABLE", "Ollama is unavailable.") from exc
+        if not isinstance(data, dict):
+            raise ProviderError("PROVIDER_INVALID_RESPONSE", "Ollama returned non-object JSON.")
+        return data