feat(enhancement): use AddEventListener check instead of OnMessage check (#989)

Co-authored-by: xiyuliu <xiyuliu@bytedance.com>

Author: vegetablechicken233

sse.go

@@ -343,8 +343,9 @@ func (es *EventSource) Get() error {
 	if isStringEmpty(es.url) {
 		return fmt.Errorf("resty:sse: event source URL is required")
 	}
-	if _, found := es.onEvent[defaultEventName]; !found {
-		return fmt.Errorf("resty:sse: OnMessage function is required")
+
+	if len(es.onEvent) == 0 {
+		return fmt.Errorf("resty:sse: At least one OnMessage/AddEventListener func is required")
 	}
 
 	// reset to begin

sse_test.go

@@ -339,7 +339,7 @@ func TestEventSourceCoverage(t *testing.T) {
 
 	es.SetURL("https://sse.dev/test")
 	err2 := es.Get()
-	assertEqual(t, "resty:sse: OnMessage function is required", err2.Error())
+	assertEqual(t, "resty:sse: At least one OnMessage/AddEventListener func is required", err2.Error())
 
 	es.OnMessage(func(a any) {}, nil)
 	es.SetURL("//res%20ty.dev")