bug: WebDAV writer rejects streaming writes larger than one buffer

### OpenDAL version or commit

v0.58.1 (the same one-shot writer is still present on main)

### Affected service or component

services/webdav against a dav-server WebDAV endpoint

### Describe the bug

The WebDAV writer is a one-shot writer (`oio::OneShotWriter`). One writer only accepts a single `write` call, so any streaming write that exceeds one internal buffer (256 KiB) fails with:

```
Unsupported (permanent) at write => OneShotWriter doesn't support multiple write
Context:
service: webdav
path: ...
size: 262144
written: 262144
```

`FuturesAsyncWriter` (returned by `Operator::writer(...).into_futures_async_write()`) flushes its 256 KiB buffer by calling `write` again, which the one-shot writer rejects. As a result, a single writer cannot upload files larger than 256 KiB.

### Steps to Reproduce

```rust
use futures::AsyncWriteExt;

let mut writer = op.writer("big.bin").await?.into_futures_async_write();
writer.write_all(&vec![0u8; 600 * 1024]).await?; // fails with the error above
writer.close().await?;
```

Reproduced against a real `dav-server` WebDAV endpoint (`dav_server::localfs::LocalFs`) through `opendal::services::Webdav`.

### Actual Behavior

The second `write` on the same writer fails with `Unsupported (permanent) at write => OneShotWriter doesn't support multiple write`.

### Expected Behavior

A single writer accepts multiple writes and uploads the data in ordered chunks (for example, the first chunk with a regular PUT and the following chunks with partial PUTs carrying `Content-Range`), and the service reports `write_can_multi = true`.

### Additional Context

`write_can_multi` is currently `false` for webdav even though `Operator::writer` hands users a writer they can write to multiple times. `write_total_max_size` / server support for partial PUTs may need to be documented as part of the fix.

### Reproduction confirmation

- [x] I reproduced this issue against the affected component. For a service-specific report, I used the actual service rather than only a mock, emulator, or source-code analysis.

### Are you willing to submit a PR to fix this bug?

- [x] Yes, I would like to submit a PR.

Start with the services/webdav writer and the Operator::writer(...).into_futures_async_write() entry point, then trace how write_can_multi is reported. Reproduce the 600 KiB streaming write against a dav-server endpoint and verify that multiple ordered writes succeed, including the documented partial-PUT and size-limit behavior.

Written by the indexing model from the issue text.