Panic on shutdown of session and server

[StarpTech]

Hi, we encountered a panic because the session channel is closed twice.

1. mcp-go/server/sse.go

   Line 206 in 71b910b

   close(session.done)

2. mcp-go/server/sse.go

   Line 309 in 71b910b

   close(session.done)

This can happen when the request context and server are shut down simultaneously. Additionally, when unregistering a session, I’d also expect the session to be aborted. This can be achieved naturally by closing the channel and listen on the close chan event. This was addressed in #128

Therefore, I saw that we have a hook for RegisterSession; it would be natural to add the counterpart UnregisterSession to it as well.

[ezynda3]

Added here 7afc2e5