mcp: use rate.Every for idiomatic rate limiting

Replace rate.Limit(1.0/opts.MinInterval.Seconds()) with
rate.Every(opts.MinInterval) which is the idiomatic way
to express a rate from a duration.

Also fix gofmt alignment in LoggingHandler struct.

Author: wucm667

mcp/logging.go

@@ -88,7 +88,7 @@ type LoggingHandler struct {
 	mu      *sync.Mutex
 	limiter *rate.Limiter // for rate-limiting
 	buf     *bytes.Buffer
-	handler         slog.Handler
+	handler slog.Handler
 }
 
 // ensureLogger returns l if non-nil, otherwise a discard logger.