fix: enforce inbox limit at model level

app/models/inbox.rb

@@ -53,6 +53,7 @@ class Inbox < ApplicationRecord
   validates :out_of_office_message, length: { maximum: Limits::OUT_OF_OFFICE_MESSAGE_MAX_LENGTH }
   validates :greeting_message, length: { maximum: Limits::GREETING_MESSAGE_MAX_LENGTH }
   validate :ensure_valid_max_assignment_limit
+  validate :ensure_inbox_limit, on: :create
 
   belongs_to :account
   belongs_to :portal, optional: true
@@ -237,6 +238,13 @@ class Inbox < ApplicationRecord
     # overridden in enterprise/app/models/enterprise/inbox.rb
   end
 
+  def ensure_inbox_limit
+    return unless account
+    return unless account.inboxes.count >= account.usage_limits[:inboxes]
+
+    errors.add(:base, 'Account inbox limit exceeded. Upgrade to a higher plan')
+  end
+
   def delete_round_robin_agents
     ::AutoAssignment::InboxRoundRobinService.new(inbox: self).clear_queue
   end