fix(whatsapp-call): look up conversation by display_id in initiate

The dashboard sends the conversation's display_id (per-account sequential
number shown in URLs) as conversation_id. The controller was calling
`conversations.find()` which resolves by primary key, so every initiate
request 404'd. Align with the rest of the codebase by using
`find_by!(display_id: ...)`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

enterprise/app/controllers/api/v1/accounts/whatsapp_calls_controller.rb

@@ -149,7 +149,7 @@ class Api::V1::Accounts::WhatsappCallsController < Api::V1::Accounts::BaseContro
   end
 
   def initiate
-    conversation = current_account.conversations.find(params[:conversation_id])
+    conversation = current_account.conversations.find_by!(display_id: params[:conversation_id])
     authorize conversation, :show?
     error = validate_whatsapp_calling(conversation)
     return render json: { error: error }, status: :unprocessable_entity if error