rename "host" to "session"

[simonpcouch]

From @shikokuchuo:

the term MCP host is also 'reserved' and actually refers to the application that 'hosts' the client:
https://modelcontextprotocol.io/introduction#general-architecture

🤪NAMING THINGS🤪

[simonpcouch]

This will still be a biiiit confusing as the server is itself an R session, but I think this will still ultimately be clearer after the change than before.