We currently return a generic instance of RequestContext without
a specialization on the Session type. This makes it impossible
for servers to typesafe call `list_roots()` and other methods.
We now return a specific instance of `RequestContext[ServerSession]`
We will be a bit more low level and expect callees to import mcp.types
instead of relying in re-exported types. This makes usage more explicit
and avoids potential collisions in mcp.server.
