fix(voice): address codex review feedback on Call-model PR

- Guard against delayed conference-start retries rolling a progressed
  call back to ringing (Conference::Manager#mark_ringing!).
- Scope conference-status webhook Call lookup to the resolved inbox
  (VoiceController#find_call_for_conference!).
- Require explicit call_sid on the agent TwiML leg and on
  ConferenceController#create/destroy so multi-call conversations can't
  attach the wrong call (no more most-recent-active fallback).
- Scope StatusUpdateService Call lookup to the webhook's account for
  defense-in-depth.
- Update UPDATE_MESSAGE_CALL_STATUS mutation to match the voice_call
  message by call_sid; helper/voice.js now passes callSid. Prevents an
  older call's status update from clobbering the newest bubble.

spec/enterprise/controllers/api/v1/accounts/conference_controller_spec.rb

@@ -91,13 +91,13 @@ RSpec.describe Api::V1::Accounts::ConferenceController, type: :request do
         expect(conference_service).to have_received(:mark_agent_joined)
       end
 
-      it 'falls back to the most recent active call on the conversation when call_sid is missing' do
+      it 'rejects the request when call_sid is missing' do
         post "/api/v1/accounts/#{account.id}/inboxes/#{voice_inbox.id}/conference",
              headers: agent.create_new_auth_token,
              params: { conversation_id: conversation.display_id }
 
-        expect(response).to have_http_status(:ok)
-        expect(conference_service).to have_received(:ensure_conference_sid)
+        expect(response).to have_http_status(:unprocessable_content)
+        expect(conference_service).not_to have_received(:ensure_conference_sid)
       end
 
       it 'does not allow accessing calls from inboxes without access' do