refactor: add jsonrpc field to base Notification class

This aligns with the TypeScript SDK where notification types extend
JSONRPCNotification and include the jsonrpc field. All notification
types now automatically have jsonrpc='2.0' via the base class.

The session.py code no longer needs to explicitly add jsonrpc since
it's included in the notification's model_dump().

Github-Issue: #1729

Author: felixweinberger

src/mcp/shared/session.py

@@ -320,8 +320,8 @@ async def send_notification(
         """
         # Some transport implementations may need to set the related_request_id
         # to attribute to the notifications to the request that triggered them.
+        # Note: notification already has jsonrpc="2.0" from base Notification class
         jsonrpc_notification = JSONRPCNotification(
-            jsonrpc="2.0",
             **notification.model_dump(by_alias=True, mode="json", exclude_none=True),
         )
         session_message = SessionMessage(  # pragma: no cover

src/mcp/types.py

@@ -108,6 +108,7 @@ class PaginatedRequest(Request[PaginatedRequestParams | None, MethodT], Generic[
 class Notification(MCPModel, Generic[NotificationParamsT, MethodT]):
     """Base class for JSON-RPC notifications."""
 
+    jsonrpc: Literal["2.0"] = "2.0"
     method: MethodT
     params: NotificationParamsT
 
@@ -142,7 +143,6 @@ class JSONRPCRequest(Request[dict[str, Any] | None, str]):
 class JSONRPCNotification(Notification[dict[str, Any] | None, str]):
     """A notification which does not expect a response."""
 
-    jsonrpc: Literal["2.0"]
     params: dict[str, Any] | None = None