Add group parameter to ScalingGroupClient.get_wsproxy_version() in SDK

The server-side handler for GET /func/scaling-groups/{sg}/wsproxy-version accepts an optional group query parameter (WsproxyVersionQueryParam.group), but the client SDK's ScalingGroupClient.get_wsproxy_version() does not expose this parameter.

File: src/ai/backend/client/v2/domains/scaling_group.py

Current signature:

async def get_wsproxy_version(self, scaling_group: str) -> GetWsproxyVersionResponse

Expected signature:

async def get_wsproxy_version(self, scaling_group: str, group: str | None = None) -> GetWsproxyVersionResponse

When group is provided, it should be passed as a query parameter (params={"group": group}).

JIRA Issue: BA-5412