refactor: import SSEError from httpx_sse public API

`SSEError` is re-exported at the `httpx_sse` package top level and
listed in `__all__` in every release >=0.4.0; the `_exceptions`
submodule is private. Importing via the public path avoids breakage if
httpx-sse reorganises its internals.

No behaviour change (same class object).

Author: maxisbey

src/mcp/client/sse.py

@@ -7,8 +7,7 @@
 import anyio
 import httpx
 from anyio.abc import TaskStatus
-from httpx_sse import aconnect_sse
-from httpx_sse._exceptions import SSEError
+from httpx_sse import SSEError, aconnect_sse
 
 from mcp import types
 from mcp.shared._context_streams import create_context_streams