fix: message_type inconsistency across message end points (#10108)

The `before_type_cast` method sometimes returns a string for
`message_type`, creating inconsistencies in different payloads. This
pull request will remove all `before_type_cast` usage and replace it
with `to_i` methods.

app/views/api/v1/widget/messages/create.json.jbuilder

@@ -2,7 +2,7 @@ json.id @message.id
 json.content @message.content
 json.inbox_id @message.inbox_id
 json.conversation_id @message.conversation.display_id
-json.message_type @message.message_type_before_type_cast
+json.message_type @message.message_type.to_i
 json.created_at @message.created_at.to_i
 json.private @message.private
 json.source_id @message.source_id

app/views/api/v1/widget/messages/index.json.jbuilder

@@ -2,7 +2,7 @@ json.payload do
   json.array! @messages do |message|
     json.id message.id
     json.content message.content
-    json.message_type message.message_type_before_type_cast
+    json.message_type message.message_type.to_i
     json.content_type message.content_type
     json.content_attributes message.content_attributes
     json.created_at message.created_at.to_i