fix: Handle JSON requests in DashboardController (#10910)

This is the error that is triggering a P0 incident in Chatwoot. 

```
DashboardController#index is missing a template for this request format and variant. request.formats: ["application/json"] request.variant: []
```

The user is calling `/app/accounts/api/v1/accounts/<account-id>/inboxes`. The URL is wrong, the requests are routed to dashboard controller as it starts with `/app/accounts`. The dashboard controller is not handling JSON requests and it creates errors. There are 312k errors over the last 2 years. Close to 50k during last 3 days.

This fix would return not_acceptable response to the attempts.

spec/controllers/dashboard_controller_spec.rb

@@ -18,6 +18,13 @@ describe '/app/login', type: :request do
     end
   end
 
+  context 'with non-HTML format' do
+    it 'returns not acceptable for JSON' do
+      get '/app/login', params: { format: 'json' }
+      expect(response).to have_http_status(:not_acceptable)
+    end
+  end
+
   # Routes are loaded once on app start
   # hence Rails.application.reload_routes! is used in this spec
   # ref : https://stackoverflow.com/a/63584877/939299