fix: Show campaigns only if the feature is enabled (#11420)

If the feature is disabled (manually or due to plan changes), the
customer cannot disable the existing campaigns. This PR would fix that.

Fixes
https://linear.app/chatwoot/issue/CW-3691/fix-disable-campaigns-on-plan-downgrade

app/controllers/api/v1/widget/campaigns_controller.rb

@@ -2,10 +2,15 @@ class Api::V1::Widget::CampaignsController < Api::V1::Widget::BaseController
   skip_before_action :set_contact
 
   def index
-    @campaigns = @web_widget
-                 .inbox
-                 .campaigns
-                 .where(enabled: true, account_id: @web_widget.inbox.account_id)
-                 .includes(:sender)
+    account = @web_widget.inbox.account
+    @campaigns = if account.feature_enabled?('campaigns')
+                   @web_widget
+                     .inbox
+                     .campaigns
+                     .where(enabled: true, account_id: account.id)
+                     .includes(:sender)
+                 else
+                   []
+                 end
   end
 end