fix(type): lifespan is partially unknown (#1389)

Co-authored-by: Kun Yang <10774721+ykun91@users.noreply.github.com>

src/fastmcp/server/http.py

@@ -72,7 +72,7 @@ _current_http_request: ContextVar[Request | None] = ContextVar(
 
 class StarletteWithLifespan(Starlette):
     @property
-    def lifespan(self) -> Lifespan:
+    def lifespan(self) -> Lifespan[Starlette]:
         return self.router.lifespan_context