fix(voice): lock webhook transitions and skip unknown terminates

- Wrap mark_outbound_accepted, accept_outbound_call, and handle_terminate in
  call.with_lock so they serialize against Whatsapp::CallService (which holds
  call.with_lock across the Meta API call). Without this, an ACCEPTED webhook
  arriving mid-terminate could overwrite the freshly-finalized terminal status
  back to in_progress and broadcast an erroneous outbound_accepted.

- Drop build_missed_inbound_call. An outbound terminate landing in the tiny
  window between provider_service.initiate_call and Call.create! had no local
  row, so the fallback materialised an inbound row with the same
  provider_call_id and tripped the unique (provider, provider_call_id) index
  on the controller's create. The "out-of-order webhook" case it protected is
  rare in practice; trade that for eliminating the false-positive collision.

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

spec/enterprise/services/whatsapp/incoming_call_service_spec.rb

@@ -114,6 +114,21 @@ describe Whatsapp::IncomingCallService do
     end
   end
 
+  describe 'terminate with no local row yet' do
+    it 'logs and skips instead of materialising an inbound missed-call row' do
+      allow(inbox.channel).to receive(:voice_enabled?).and_return(true)
+      allow(Rails.logger).to receive(:warn)
+      allow(ActionCable.server).to receive(:broadcast)
+
+      params = call_payload(event: 'terminate', duration: 0, terminate_reason: 'no_answer')
+
+      expect { described_class.new(inbox: inbox, params: params).perform }
+        .not_to change(Call, :count)
+      expect(Rails.logger).to have_received(:warn).with(/Terminate for unknown call/)
+      expect(ActionCable.server).not_to have_received(:broadcast)
+    end
+  end
+
   describe 'outbound connect with no local row yet' do
     it 'does not mint an inbound call when sdp_type is answer' do
       allow(inbox.channel).to receive(:voice_enabled?).and_return(true)