[MM-62427] Add message attachments validation (#30180)

* Add message attachments validation

* Add props validation

* Validate slack attachment fields

* Update tests and library usage

* Improve interactive dialog error for length checks

* Allow predefined colors for slack attachments

* Fix TestPostAction

* Use const for data source

* Add tests

* Cleanup unused props

* Add happy path tests

* lint fixes

* Add validation for PostActionOptions

server/public/pluginapi/experimental/flow/handler.go

@@ -196,7 +196,7 @@ func (f *Flow) handle(
 }
 
 func (f *Flow) processButtonPostActions(post *model.Post) {
-	attachments, ok := post.GetProp("attachments").([]*model.SlackAttachment)
+	attachments, ok := post.GetProp(model.PostPropsAttachments).([]*model.SlackAttachment)
 	if !ok || len(attachments) == 0 {
 		return
 	}

server/public/pluginapi/experimental/flow/step.go

@@ -145,7 +145,7 @@ func (s Step) render(f *Flow, done bool, selectedButton int) (*model.Post, bool,
 
 	buttons := processButtons(s.buttons, f.state.AppState)
 
-	attachments, ok := post.GetProp("attachments").([]*model.SlackAttachment)
+	attachments, ok := post.GetProp(model.PostPropsAttachments).([]*model.SlackAttachment)
 	if !ok || len(attachments) != 1 {
 		return nil, false, errors.New("expected 1 slack attachment")
 	}
@@ -224,6 +224,7 @@ func processDialog(in *model.Dialog, state State) model.Dialog {
 
 func renderButton(b Button, stepName Name, i int, state State) *model.PostAction {
 	return &model.PostAction{
+		Type:     model.PostActionTypeButton,
 		Name:     formatState(b.Name, state),
 		Disabled: b.Disabled,
 		Style:    string(b.Color),