Add tests for AddAlternativeTextTemplate method

Include various scenarios like default, with existing body string, nil template, and invalid template. These tests help in validating the addition of alternative text templates to messages.

msg_test.go

@@ -4365,6 +4365,114 @@ func TestMsg_AddAlternativeHTMLTemplate(t *testing.T) {
 	})
 }
 
+func TestMsg_AddAlternativeTextTemplate(t *testing.T) {
+	tplString := `Teststring: {{.teststring}}`
+	invalidTplString := `Teststring: {{call $.invalid .teststring}}`
+	data := map[string]interface{}{"teststring": "this is a test"}
+	textTpl, err := ttpl.New("texttpl").Parse(tplString)
+	if err != nil {
+		t.Fatalf("failed to parse Text template: %s", err)
+	}
+	invalidTpl, err := ttpl.New("texttpl").Parse(invalidTplString)
+	if err != nil {
+		t.Fatalf("failed to parse invalid Text template: %s", err)
+	}
+	t.Run("AddAlternativeTextTemplate default", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		if err = message.AddAlternativeTextTemplate(textTpl, data); err != nil {
+			t.Fatalf("failed to set body text template: %s", err)
+		}
+		parts := message.GetParts()
+		if len(parts) != 1 {
+			t.Fatalf("expected 1 part, got: %d", len(parts))
+		}
+		if parts[0] == nil {
+			t.Fatal("expected part to be not nil")
+		}
+		if parts[0].contentType != TypeTextPlain {
+			t.Errorf("expected contentType to be %s, got: %s", TypeTextPlain, parts[0].contentType)
+		}
+		messageBuf := bytes.NewBuffer(nil)
+		_, err = parts[0].writeFunc(messageBuf)
+		if err != nil {
+			t.Errorf("writeFunc failed: %s", err)
+		}
+		if !strings.EqualFold(messageBuf.String(), "Teststring: this is a test") {
+			t.Errorf("expected message body to be %s, got: %s", "Teststring: this is a test", messageBuf.String())
+		}
+	})
+	t.Run("AddAlternativeTextTemplate with body string", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		message.SetBodyString(TypeTextPlain, "body string")
+		if err = message.AddAlternativeTextTemplate(textTpl, data); err != nil {
+			t.Fatalf("failed to set body text template: %s", err)
+		}
+		parts := message.GetParts()
+		if len(parts) != 2 {
+			t.Fatalf("expected 2 part, got: %d", len(parts))
+		}
+		if parts[0] == nil || parts[1] == nil {
+			t.Fatal("expected part to be not nil")
+		}
+		if parts[0].contentType != TypeTextPlain {
+			t.Errorf("expected contentType to be %s, got: %s", TypeTextPlain, parts[0].contentType)
+		}
+		if parts[1].contentType != TypeTextPlain {
+			t.Errorf("expected contentType to be %s, got: %s", TypeTextPlain, parts[1].contentType)
+		}
+		messageBuf := bytes.NewBuffer(nil)
+		_, err = parts[0].writeFunc(messageBuf)
+		if err != nil {
+			t.Errorf("writeFunc failed: %s", err)
+		}
+		if !strings.EqualFold(messageBuf.String(), "body string") {
+			t.Errorf("expected message body to be %s, got: %s", "body string", messageBuf.String())
+		}
+		messageBuf.Reset()
+		_, err = parts[1].writeFunc(messageBuf)
+		if err != nil {
+			t.Errorf("writeFunc failed: %s", err)
+		}
+		if !strings.EqualFold(messageBuf.String(), "Teststring: this is a test") {
+			t.Errorf("expected message body to be %s, got: %s", "Teststring: this is a test", messageBuf.String())
+		}
+	})
+	t.Run("AddAlternativeTextTemplate with nil tpl", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		err = message.AddAlternativeTextTemplate(nil, nil)
+		if err == nil {
+			t.Fatal("expected error, got nil")
+		}
+		if !strings.EqualFold(err.Error(), errTplPointerNil) {
+			t.Errorf("expected error to be %s, got: %s", errTplPointerNil, err.Error())
+		}
+	})
+	t.Run("AddAlternativeTextTemplate with invalid tpl", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		err = message.AddAlternativeTextTemplate(invalidTpl, data)
+		if err == nil {
+			t.Fatal("expected error, got nil")
+		}
+		expectErr := `failed to execute template: template: texttpl:1:14: executing "texttpl" at <call $.invalid ` +
+			`.teststring>: error calling call: call of nil`
+		if !strings.EqualFold(err.Error(), expectErr) {
+			t.Errorf("expected error to be %s, got: %s", expectErr, err.Error())
+		}
+	})
+}
+
 /*
 // TestNewMsgWithMiddleware tests WithMiddleware