refactor(voice): drop content_attributes.data from voice_call messages

Nothing reads it anymore — frontend uses message.call, CallMessageBuilder
uses call.message (the FK), and the jsonb fallback finder was dead code
(call.message_id is always linked immediately after message creation, in
the same transaction).

New voice_call messages are now created with blank content_attributes.
Existing rows keep their legacy payload; nothing reads it, so it's inert.
Safe to backfill with `Message.voice_calls.update_all(content_attributes: {})`
if you want a tidy table.

enterprise/app/services/voice/call_message_builder.rb

@@ -8,26 +8,18 @@ class Voice::CallMessageBuilder
   end
 
   def perform!
-    find_message || create_message!
+    call.message || create_message!
   end
 
   private
 
   attr_reader :call
 
-  def find_message
-    return call.message if call.message_id.present?
-
-    call.conversation.messages.voice_calls
-        .find_by("content_attributes -> 'data' ->> 'call_sid' = ?", call.provider_call_id)
-  end
-
   def create_message!
     params = {
       content: 'Voice Call',
       message_type: call.outgoing? ? 'outgoing' : 'incoming',
-      content_type: 'voice_call',
-      content_attributes: { 'data' => { 'call_sid' => call.provider_call_id } }
+      content_type: 'voice_call'
     }
     Messages::MessageBuilder.new(sender, call.conversation, params).perform
   end

spec/enterprise/services/voice/inbound_call_builder_spec.rb

@@ -48,7 +48,6 @@ RSpec.describe Voice::InboundCallBuilder do
         expect(voice_message).to be_present
         expect(voice_message.message_type).to eq('incoming')
         expect(call.message_id).to eq(voice_message.id)
-        expect(voice_message.content_attributes['data']['call_sid']).to eq(call_sid)
         expect(voice_message.call).to eq(call)
       end
     end

spec/enterprise/services/voice/outbound_call_builder_spec.rb

@@ -40,7 +40,6 @@ RSpec.describe Voice::OutboundCallBuilder do
         voice_message = call.conversation.messages.voice_calls.last
         expect(call.message_id).to eq(voice_message.id)
         expect(voice_message.message_type).to eq('outgoing')
-        expect(voice_message.content_attributes['data']['call_sid']).to eq(call_sid)
         expect(voice_message.call).to eq(call)
       end
     end