feat: Add inbox-label matrix report endpoint (#13394)

This PR added new API endpoint GET
/api/v2/accounts/:account_id/reports/inbox_label_matrix that returns
conversation counts grouped by inbox and label in a matrix format.
Supports optional filtering by date range, inbox_ids, and label_ids.

---------

Co-authored-by: Pranav <pranav@chatwoot.com>

config/routes.rb

@@ -444,6 +444,7 @@ Rails.application.routes.draw do
               get :conversations_summary
               get :conversation_traffic
               get :bot_metrics
+              get :inbox_label_matrix
             end
           end
           resource :year_in_review, only: [:show]