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`
