[Feature] OSS 上传/下载支持 timeout 配置

**Feature Category**

- [ ] Sandbox
- [ ] Actions
- [ ] Deployments
- [x] SDK & API
- [ ] Envhub
- [ ] CLI
- [ ] Performance & Optimization
- [ ] Documentation & Examples

**Problem Statement**

使用 OSS 模式上传/下载文件时，当文件较大或网络较慢时，可能触发 oss2 SDK 的默认超时，导致传输失败。当前 `_setup_oss()` 方法初始化 OSS 客户端时没有设置 timeout 参数，无法根据实际需求调整超时时间。

```python
# 当前代码缺少 timeout 配置
self._oss_bucket = oss2.Bucket(
auth=auth,
endpoint=env_vars.ROCK_OSS_BUCKET_ENDPOINT,
bucket_name=env_vars.ROCK_OSS_BUCKET_NAME,
region=env_vars.ROCK_OSS_BUCKET_REGION,
# 缺少 timeout 配置！
)
```

**Proposed Solution**

同时支持**环境变量**和**函数参数**两种方式配置 timeout，优先级为：函数参数 > 环境变量 > SDK 默认值（300000ms = 5分钟）

**Detailed Feature Description**

### 1. 环境变量支持

在 `rock/env_vars.py` 中添加：

```python
"ROCK_OSS_TIMEOUT": lambda: int(os.getenv("ROCK_OSS_TIMEOUT", "300000")), # 默认 5 分钟
```

### 2. 修改 `_setup_oss()` 方法

```python
async def _setup_oss(self, timeout: int | None = None) -> OssSetupResponse:
try:
credentials = await self._get_oss_sts_credentials()
auth = oss2.StsAuth(
credentials["AccessKeyId"],
credentials["AccessKeySecret"],
credentials["SecurityToken"],
)

# 优先级：参数 > 环境变量 > 默认值
oss_timeout = timeout or env_vars.ROCK_OSS_TIMEOUT

self._oss_bucket = oss2.Bucket(
auth=auth,
endpoint=env_vars.ROCK_OSS_BUCKET_ENDPOINT,
bucket_name=env_vars.ROCK_OSS_BUCKET_NAME,
region=env_vars.ROCK_OSS_BUCKET_REGION,
connect_timeout=oss_timeout / 1000, # oss2 使用秒为单位
)
except Exception as e:
return OssSetupResponse(success=False, message=f"Failed to setup oss bucket: {e}")
return OssSetupResponse(success=True, message="Successfully setup oss bucket")
```

### 3. 公开方法支持 timeout 参数

```python
# upload_by_path
async def upload_by_path(
self,
source_path: str,
target_path: str,
upload_mode: UploadMode = "auto",
timeout: int | None = None, # 新增
) -> UploadResponse

# download_file
async def download_file(
self,
remote_path: str,
local_path: str,
timeout: int | None = None, # 新增
) -> DownloadFileResponse
```

### 4. 使用示例

```python
from rock.sdk import Sandbox

# 方式 1：环境变量
# export ROCK_OSS_TIMEOUT=600000

async with Sandbox() as sandbox:
# 方式 2：函数参数
await sandbox.upload_by_path(
"/local/file.pdf",
"/remote/file.pdf",
"oss",
timeout=600000 # 10 分钟
)
```

### 5. 参考

TypeScript SDK 已在 v1.3.8 版本中实现此功能，可作为参考实现。

Start with rock/env_vars.py and locate _setup_oss(), upload_by_path(), and download_file(). Trace how OSS setup and these public methods currently pass configuration, then implement the documented timeout precedence and unit conversion. Done means both environment-variable and per-call timeout options work for OSS transfers while retaining the SDK default.

Written by the indexing model from the issue text.