chore: Discard activities from email notifiable messages (#3989)

spec/models/message_spec.rb

@@ -111,6 +111,7 @@ RSpec.describe Message, type: :model do
 
     it 'return false if private message' do
       message.private = true
+      message.message_type = 'outgoing'
       expect(message.email_notifiable_message?).to be false
     end
 
@@ -120,6 +121,12 @@ RSpec.describe Message, type: :model do
       expect(message.email_notifiable_message?).to be false
     end
 
+    it 'return false if activity message' do
+      message.private = false
+      message.message_type = 'activity'
+      expect(message.email_notifiable_message?).to be false
+    end
+
     it 'return false if message type is template and content type is not input_csat or text' do
       message.private = false
       message.message_type = 'template'