fix: Use message sender ID in the analytics for first response time (#9605)

`conversation.assignee_id` was taken into consideration for first
response time calculation. There was one flaw in this approach. A lot of
customer support agents respond to the conversation first before
assigning it to themselves. This makes the analytics broken.

To fix this, we will take the message sender instead of assignee in
calculations, which should solve the issue with metric.

Fixes
https://linear.app/chatwoot/issue/CW-3375/first-response-reporting-events-use-assignee-id-instead-of-agent-who

app/listeners/reporting_event_listener.rb

@@ -34,7 +34,7 @@ class ReportingEventListener < BaseListener
                                               message.created_at),
       account_id: conversation.account_id,
       inbox_id: conversation.inbox_id,
-      user_id: conversation.assignee_id,
+      user_id: message.sender_id,
       conversation_id: conversation.id,
       event_start_time: last_non_human_activity(conversation),
       event_end_time: message.created_at

spec/listeners/reporting_event_listener_spec.rb

@@ -99,7 +99,9 @@ describe ReportingEventListener do
       it 'creates first_response event with business hour value' do
         event = Events::Base.new('first.reply.created', Time.zone.now, message: new_message)
         listener.first_reply_created(event)
-        expect(account.reporting_events.where(name: 'first_response')[0]['value_in_business_hours']).to be 144_000.0
+        reporting_event = account.reporting_events.where(name: 'first_response').first
+        expect(reporting_event.value_in_business_hours).to be 144_000.0
+        expect(reporting_event.user_id).to be new_message.sender_id
       end
     end