MCPClient StreamableHTTP import breaks with mcp>=2.0.0 (streamablehttp_client renamed to streamable_http_client)

### Describe the bug

`MCPClient`'s StreamableHTTP connection path still imports the pre-2.0 `mcp` SDK symbol name, which was renamed/removed in `mcp==2.0.0`:

https://github.com/huggingface/huggingface_hub/blob/main/src/huggingface_hub/inference/_mcp/mcp_client.py#L186

```python
elif type == "http":
    # Handle StreamableHTTP server
    from mcp.client.streamable_http import streamablehttp_client
```

As of `mcp==2.0.0`, this function was renamed to `streamable_http_client` (with an underscore between `streamable` and `http`) — `streamablehttp_client` no longer exists. Confirmed against the latest `huggingface_hub` release (1.28.0), the import is unchanged.

### Reproduction

```python
from huggingface_hub import MCPClient

client = MCPClient()
await client.add_mcp_server(type="http", url="http://localhost:8000/mcp")
```

with `mcp>=2.0.0` installed raises:

```
ImportError: cannot import name 'streamablehttp_client' from 'mcp.client.streamable_http'
```

### Expected behavior

The import should use the current `mcp` SDK name:

```python
from mcp.client.streamable_http import streamable_http_client
```

(possibly with a fallback for older `mcp<2.0.0` installs, since `huggingface_hub` doesn't currently pin an `mcp` version).

### Environment

- `huggingface_hub` version: 1.28.0 (latest, confirmed via the published wheel; also reproduced with 1.15.0/0.31.2)
- `mcp` version: 2.0.0+ (any version where the rename applies)
- Discovered while debugging an unrelated `ImportError` in a downstream project (`mcp` is pulled in independently, not as a declared `huggingface_hub` dependency)

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