mcp: gracefully handle a nil server in handlers (#164)

If the getServer function passed to NewSSEHandler or
NewStreamableHTTPHandler returns nil, serve a 400 instead of panicking.

Fixes #161.

examples/sse/main.go

@@ -53,5 +53,5 @@ func main() {
 			return nil
 		}
 	})
-	http.ListenAndServe(*httpAddr, handler)
+	log.Fatal(http.ListenAndServe(*httpAddr, handler))
 }