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/http_base.py

@@ -0,0 +1,82 @@
+import json
+from collections.abc import AsyncIterator
+from datetime import datetime, timezone
+
+from app.contracts import ModelEvent, ModelEventType, ModelRequest
+from app.providers.base import ProviderError, ProviderTurn
+
+
+class TurnStreamingMixin:
+    async def complete(self, request: ModelRequest) -> ProviderTurn:
+        raise NotImplementedError
+
+    async def stream(self, request: ModelRequest) -> AsyncIterator[ModelEvent]:
+        try:
+            turn = await self.complete(request)
+            sequence = 0
+            if turn.text:
+                yield ModelEvent(
+                    event=ModelEventType.text_delta,
+                    sequence=sequence,
+                    data={"text": turn.text},
+                    timestamp=datetime.now(timezone.utc),
+                )
+                sequence += 1
+            for call in turn.tool_calls:
+                yield ModelEvent(
+                    event=ModelEventType.tool_call_start,
+                    sequence=sequence,
+                    data={
+                        "tool_call_id": call.tool_call_id,
+                        "name": call.name,
+                        "arguments": call.arguments,
+                    },
+                    timestamp=datetime.now(timezone.utc),
+                )
+                sequence += 1
+                yield ModelEvent(
+                    event=ModelEventType.tool_call_end,
+                    sequence=sequence,
+                    data={"tool_call_id": call.tool_call_id},
+                    timestamp=datetime.now(timezone.utc),
+                )
+                sequence += 1
+            yield ModelEvent(
+                event=ModelEventType.usage,
+                sequence=sequence,
+                data={
+                    "input_tokens": turn.input_tokens,
+                    "output_tokens": turn.output_tokens,
+                },
+                timestamp=datetime.now(timezone.utc),
+            )
+            yield ModelEvent(
+                event=ModelEventType.done,
+                sequence=sequence + 1,
+                timestamp=datetime.now(timezone.utc),
+            )
+        except ProviderError as exc:
+            yield ModelEvent(
+                event=ModelEventType.error,
+                data={"code": exc.code, "message": exc.message},
+                timestamp=datetime.now(timezone.utc),
+            )
+            yield ModelEvent(
+                event=ModelEventType.done,
+                sequence=1,
+                timestamp=datetime.now(timezone.utc),
+            )
+
+
+def decode_tool_arguments(value: object) -> dict[str, object]:
+    if isinstance(value, dict):
+        return value
+    if not isinstance(value, str):
+        raise ProviderError("PROVIDER_INVALID_RESPONSE", "Tool arguments are not JSON.")
+    try:
+        decoded = json.loads(value)
+    except json.JSONDecodeError as exc:
+        raise ProviderError("PROVIDER_INVALID_RESPONSE", "Tool arguments are invalid JSON.") from exc
+    if not isinstance(decoded, dict):
+        raise ProviderError("PROVIDER_INVALID_RESPONSE", "Tool arguments must be an object.")
+    return decoded