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.

spec/controllers/public/api/v1/inbox/messages_controller_spec.rb

@@ -26,6 +26,7 @@ RSpec.describe 'Public Inbox Contact Conversation Messages API', type: :request
       expect(response).to have_http_status(:success)
       data = response.parsed_body
       expect(data['content']).to eq('hello')
+      expect(data['message_type']).to eq(0)
     end
 
     it 'does not create the message' do