Add 'mark thread as done' functionality (google#3265)

Fixes google#3264.

github/activity_notifications.go

@@ -178,6 +178,23 @@ func (s *ActivityService) MarkThreadRead(ctx context.Context, id string) (*Respo
 	return s.client.Do(ctx, req, nil)
 }
 
+// MarkThreadDone marks the specified thread as done.
+// Marking a thread as "done" is equivalent to marking a notification in your notification inbox on GitHub as done.
+//
+// GitHub API docs: https://docs.github.com/rest/activity/notifications#mark-a-thread-as-done
+//
+//meta:operation DELETE /notifications/threads/{thread_id}
+func (s *ActivityService) MarkThreadDone(ctx context.Context, id int64) (*Response, error) {
+	u := fmt.Sprintf("notifications/threads/%v", id)
+
+	req, err := s.client.NewRequest("DELETE", u, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	return s.client.Do(ctx, req, nil)
+}
+
 // GetThreadSubscription checks to see if the authenticated user is subscribed
 // to a thread.
 //

github/activity_notifications_test.go

@@ -208,6 +208,32 @@ func TestActivityService_MarkThreadRead(t *testing.T) {
 	})
 }
 
+func TestActivityService_MarkThreadDone(t *testing.T) {
+	client, mux, _, teardown := setup()
+	defer teardown()
+
+	mux.HandleFunc("/notifications/threads/1", func(w http.ResponseWriter, r *http.Request) {
+		testMethod(t, r, "DELETE")
+		w.WriteHeader(http.StatusResetContent)
+	})
+
+	ctx := context.Background()
+	_, err := client.Activity.MarkThreadDone(ctx, 1)
+	if err != nil {
+		t.Errorf("Activity.MarkThreadDone returned error: %v", err)
+	}
+
+	const methodName = "MarkThreadDone"
+	testBadOptions(t, methodName, func() (err error) {
+		_, err = client.Activity.MarkThreadDone(ctx, 0)
+		return err
+	})
+
+	testNewRequestAndDoFailure(t, methodName, client, func() (*Response, error) {
+		return client.Activity.MarkThreadDone(ctx, 1)
+	})
+}
+
 func TestActivityService_GetThreadSubscription(t *testing.T) {
 	client, mux, _, teardown := setup()
 	defer teardown()