Anything MCPAdapter accepts as its target.
Every transport fastmcp.Client accepts, plus a pre-built fastmcp.Client.
A str target is the one member narrowed relative to fastmcp.Client: it must
be an http/https URL. Local servers are reached through Path, an explicit
transport, or MCPConfig — see MCPAdapter.
ClientGroup is the one member fastmcp.Client does not accept at all, since a
group is a peer of Client rather than a transport it could wrap.
