`virtual_hosted_style_request=True` needs the bucket in the endpoint — undocumented, and no error when it's missing

**obstore 0.11.0, Python 3.13**

obstore and boto3 interpret the endpoint differently in virtual-hosted mode:

| | boto3 (`addressing_style="virtual"`) | obstore (`virtual_hosted_style_request=True`) |
| ------------------------ | ------------------------------------- | --------------------------------------------- |
| endpoint you pass | service root (`https://cwobject.com`) | full bucket host (`https://my-bucket.cwobject.com`) |
| bucket host derived by | client (prepends `my-bucket.`) | caller |

Coming from boto3, passing the service root is the natural thing to do:

```python
store = S3Store(
"my-bucket",
endpoint="https://cwobject.com", # service root, boto3-style
virtual_hosted_style_request=True,
)
obstore.list(store).collect()
```

The bucket then appears nowhere in the request. This makes env-driven args for endpoint not-reusable (and currently not overridable https://github.com/developmentseed/obstore/issues/753)

Start with the Python S3Store endpoint handling and the obstore.list(store) request path described in the issue; compare virtual-hosted behavior with boto3's addressing style. Done means the bucket is present in requests when given a service-root endpoint, or a clear error explains the required full bucket host, with the undocumented behavior covered by a regression test.

Written by the indexing model from the issue text.