fix(security): Enforce admin authorization on custom attribute definitions API (#14392)

Custom attribute definitions can now only be created, edited, or deleted
by administrators, matching the existing settings UI restriction.
Previously, an agent could call the `custom_attribute_definitions` API
directly and modify account configuration that they couldn't reach
through the dashboard — a Broken Access Control vulnerability reported
externally.

Fixes
https://linear.app/chatwoot/issue/CW-7038/broken-access-control-on-custom-attribute-definitions-api

## How to test

1. Sign in as an agent.
2. Try to create a custom attribute by calling `POST
/api/v1/accounts/<id>/custom_attribute_definitions` directly (the
settings page is hidden for agents — use curl with the agent's
`api_access_token`).
3. Expect `401 Unauthorized` with body `{"error":"You are not authorized
to do this action"}`. Repeat for `PATCH` and `DELETE`.
4. Sign in as an administrator and confirm create/edit/delete still work
from Settings → Custom Attributes.
5. As either role, the listing endpoint (`GET
.../custom_attribute_definitions`) should still succeed — agents need
this to render attributes in conversation and contact panels.

Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>

app/controllers/api/v1/accounts/custom_attribute_definitions_controller.rb

@@ -1,6 +1,7 @@
 class Api::V1::Accounts::CustomAttributeDefinitionsController < Api::V1::Accounts::BaseController
   before_action :fetch_custom_attributes_definitions, except: [:create]
   before_action :fetch_custom_attribute_definition, only: [:show, :update, :destroy]
+  before_action :check_authorization
   DEFAULT_ATTRIBUTE_MODEL = 'conversation_attribute'.freeze
 
   def index; end