don't automatically dispatch to a busy session

I just ran into a (simplified) situation where I had 2 R sessions running, both configured with mcp_session() via .Rprofile.

• Session 1), the first one to be connected with mcp_session(), is running long-running code and has been / will be busy for hours
• Session 2) is mostly idle

If I explicitly ask to list_r_sessions() and then select 2), the server runs fine. Otherwise, the request is automatically dispatched to 1), where it fails.

Notably, when I list_r_sessions(), 1) doesn't even show up—this makes sense to me and doesn't require any changes—but I think our default session should change from:

• Before: The lowest-numbered session (or the server itself if there is none), to
• After: The lowest-numbered session that is actually responsive, e.g. the smallest index that would appear in list_r_session() if called (or the server itself if there is none)

Start by tracing the default dispatch behavior from mcp_session() and compare it with the responsive-session filtering used by list_r_sessions(). Done means requests choose the lowest-numbered responsive session, while retaining the server fallback when no such session exists; verify both busy and idle R sessions.