fix(voice): keep terminate handling idempotent for ended calls

A retried terminate webhook for an answered short call (duration=0)
would re-enter handle_terminate, find call.in_progress? false, fall
through to no_answer, and corrupt a completed call's status. Bail when
the call is already terminal so retries don't recompute the status.

enterprise/app/services/whatsapp/incoming_call_service.rb

@@ -52,6 +52,10 @@ class Whatsapp::IncomingCallService
   def handle_terminate(payload)
     call = Call.whatsapp.find_by(provider_call_id: payload[:id])
     return unless call
+    # Webhook retries can re-deliver terminate after we've already finalized the
+    # call; don't recompute status or a duration=0 retry can flip a completed
+    # short call back to no_answer.
+    return if call.terminal?
 
     duration = payload[:duration]&.to_i
     status = answered?(call, duration) ? 'completed' : 'no_answer'

spec/enterprise/services/whatsapp/incoming_call_service_spec.rb

@@ -101,6 +101,17 @@ describe Whatsapp::IncomingCallService do
       described_class.new(inbox: inbox, params: params).perform
       expect(call.reload.status).to eq('no_answer')
     end
+
+    it 'is a no-op when the call is already terminal so retries cannot flip a completed call to no_answer' do
+      call.update!(status: 'completed', duration_seconds: 5, direction: :outgoing, accepted_by_agent: nil)
+      allow(ActionCable.server).to receive(:broadcast)
+      params = call_payload(event: 'terminate', duration: 0, terminate_reason: 'completed_normally')
+
+      described_class.new(inbox: inbox, params: params).perform
+
+      expect(call.reload).to have_attributes(status: 'completed', duration_seconds: 5)
+      expect(ActionCable.server).not_to have_received(:broadcast)
+    end
   end
 
   describe 'duplicate inbound connect' do