fix: enforce inbox limits at model level (#14949)

Fixes https://linear.app/chatwoot/issue/CW-7559/inbox-limit-abuse

## Why

The regular inbox API checked limits in the controller, but WhatsApp
embedded signup creates inboxes through a service using `Inbox.create!`.
That let Enterprise account inbox limits be skipped for embedded signup.

## What this change does

- Adds an Inbox create-time validation hook in OSS and implements the
limit check in the Enterprise Inbox module.
- Removes the duplicate controller/helper limit check so the model is
the single enforcement point.
- Preserves the existing `402 Payment Required` API response for account
inbox limit failures.
- Keeps updates to existing inboxes allowed when an account is already
at its inbox limit.

## Validation

- `bundle exec rspec
spec/controllers/api/v1/accounts/inboxes_controller_spec.rb
spec/enterprise/models/inbox_spec.rb`

app/helpers/api/v1/inboxes_helper.rb

@@ -114,10 +114,4 @@ module Api::V1::InboxesHelper
       'sms' => Current.account.sms_channels
     }[permitted_params[:channel][:type]]
   end
-
-  def validate_limit
-    return unless Current.account.inboxes.count >= Current.account.usage_limits[:inboxes]
-
-    render_payment_required('Account limit exceeded. Upgrade to a higher plan')
-  end
 end