[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
 	}