The client's message_handler is only ever called with a validated
ServerNotification or a transport-level Exception; server-initiated
requests are answered by the typed callbacks. The RequestResponder arm
of the MessageHandlerFnT union was a v1 leftover with a typing-only
stub behind it, so every handler had to annotate a case the session
never delivers.
Type the parameter as an exported IncomingMessage alias
(ServerNotification | Exception) in mcp.client, delete the dead
RequestResponder stub, and drop the now-empty mcp.shared.session
compatibility module (ProgressFnT already lives in mcp.shared.dispatcher).
Max Isbey