Add tests for Msg.AddToFormat

Introduced new test cases to validate Msg.AddToFormat functionality with both valid and invalid email addresses. Ensured proper error handling and address formatting in the message headers.

msg_test.go

@@ -1430,6 +1430,70 @@ func TestMsg_AddTo(t *testing.T) {
 	})
 }
 
+func TestMsg_AddToFormat(t *testing.T) {
+	t.Run("AddToFormat with valid addresses", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		if err := message.To("toni.tester@example.com"); err != nil {
+			t.Fatalf("failed to set To: %s", err)
+		}
+		if err := message.AddToFormat("Tina Tester", "tina.tester@example.com"); err != nil {
+			t.Fatalf("failed to set additional To: %s", err)
+		}
+		addresses, ok := message.addrHeader[HeaderTo]
+		if !ok {
+			t.Fatalf("failed to set To, addrHeader field is not set")
+		}
+		if len(addresses) != 2 {
+			t.Fatalf("failed to set additional To, addrHeader value count is: %d, want: 1", len(addresses))
+		}
+		if addresses[0].Address != "toni.tester@example.com" {
+			t.Errorf("failed to set To, addrHeader value is %s, want: %s", addresses[0].Address,
+				"toni.tester@example.com")
+		}
+		if addresses[0].String() != "<toni.tester@example.com>" {
+			t.Errorf("failed to set To, addrHeader value is %s, want: %s", addresses[0].String(),
+				"<toni.tester@example.com>")
+		}
+		if addresses[0].Name != "" {
+			t.Errorf("failed to set To, addrHeader name is %s, want: empty", addresses[0].Name)
+		}
+		if addresses[1].Address != "tina.tester@example.com" {
+			t.Errorf("failed to set additional To, addrHeader value is %s, want: %s", addresses[1].Address,
+				"tina.tester@example.com")
+		}
+		if addresses[1].String() != `"Tina Tester" <tina.tester@example.com>` {
+			t.Errorf("failed to set additional To, addrHeader value is %s, want: %s", addresses[1].String(),
+				"<tina.tester@example.com>")
+		}
+		if addresses[1].Name != "Tina Tester" {
+			t.Errorf("failed to set additional To, addrHeader name is %s, want: %s", addresses[1].Name,
+				"Tina Tester")
+		}
+	})
+	t.Run("AddToFormat with invalid address", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		if err := message.To("toni.tester@example.com"); err != nil {
+			t.Fatalf("failed to set To: %s", err)
+		}
+		if err := message.AddToFormat("Invalid", "invalid"); err == nil {
+			t.Errorf("AddToFormat should fail with invalid address")
+		}
+		addresses, ok := message.addrHeader[HeaderTo]
+		if !ok {
+			t.Fatalf("failed to set To, addrHeader field is not set")
+		}
+		if len(addresses) != 1 {
+			t.Fatalf("failed to set To, addrHeader value count is: %d, want: 1", len(addresses))
+		}
+	})
+}
+
 /*
 // TestNewMsgWithMiddleware tests WithMiddleware