[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/enterprise/elasticsearch/common/common.go

@@ -110,7 +110,7 @@ func ESPostFromPostForIndexing(post *model.PostForIndexing) *ESPost {
 
 	var searchAttachments []string
 
-	if attachments := post.GetProp("attachments"); attachments != nil {
+	if attachments := post.GetProp(model.PostPropsAttachments); attachments != nil {
 		attachmentsInterfaceArray, ok := attachments.([]any)
 		if ok {
 			for _, attachment := range attachmentsInterfaceArray {