Return a specific server session instance of request context

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]`

Author: dsp-ant

src/mcp/server/__init__.py

@@ -16,7 +16,7 @@
 
 logger = logging.getLogger(__name__)
 
-request_ctx: contextvars.ContextVar[RequestContext] = contextvars.ContextVar(
+request_ctx: contextvars.ContextVar[RequestContext[ServerSession]] = contextvars.ContextVar(
     "request_ctx"
 )
 
@@ -115,7 +115,7 @@ def get_capabilities(
         )
 
     @property
-    def request_context(self) -> RequestContext:
+    def request_context(self) -> RequestContext[ServerSession]:
         """If called outside of a request context, this will raise a LookupError."""
         return request_ctx.get()