fix(client): export status_error_data in _transport's __all__

The module keeps an exhaustive export list; the helper is imported by
sse.py and streamable_http.py.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AuJi8kEB3bhikW2pzbmhUL

Author: claude

src/mcp/client/_transport.py

@@ -10,7 +10,7 @@
 from mcp.shared._stream_protocols import ReadStream, WriteStream
 from mcp.shared.message import SessionMessage
 
-__all__ = ["ReadStream", "WriteStream", "Transport", "TransportStreams"]
+__all__ = ["ReadStream", "WriteStream", "Transport", "TransportStreams", "status_error_data"]
 
 TransportStreams = tuple[ReadStream[SessionMessage | Exception], WriteStream[SessionMessage]]