fix: set default content type as text for message (#3060)

Fixes #2974

app/models/message.rb

@@ -5,7 +5,7 @@
 #  id                  :integer          not null, primary key
 #  content             :text
 #  content_attributes  :json
-#  content_type        :integer          default("text")
+#  content_type        :integer          default("text"), not null
 #  external_source_ids :jsonb
 #  message_type        :integer          not null
 #  private             :boolean          default(FALSE)
@@ -32,10 +32,13 @@ class Message < ApplicationRecord
   include MessageFilterHelpers
   NUMBER_OF_PERMITTED_ATTACHMENTS = 15
 
+  before_validation :ensure_content_type
+
   validates :account_id, presence: true
   validates :inbox_id, presence: true
   validates :conversation_id, presence: true
   validates_with ContentAttributeValidator
+  validates :content_type, presence: true
 
   # when you have a temperory id in your frontend and want it echoed back via action cable
   attr_accessor :echo_id
@@ -133,6 +136,10 @@ class Message < ApplicationRecord
 
   private
 
+  def ensure_content_type
+    self.content_type ||= Message.content_types[:text]
+  end
+
   def execute_after_create_commit_callbacks
     # rails issue with order of active record callbacks being executed https://github.com/rails/rails/issues/20911
     reopen_conversation