feat(voice): Wire Twilio voice flow through unified call model (#14091)

Twilio voice now uses first-class `Call` records as the source of truth
for call state, instead of storing it on
`conversation.additional_attributes` and `conversation.identifier`. Each
call gets its own record, its own `voice_call` bubble matched by
`call_sid`, and its own conference name keyed off `Call.id`. Multiple
calls on the same conversation (for `lock_to_single_conversation`
inboxes) now work correctly, and the conversation card stays in sync
with the real latest message.
Fixes https://linear.app/chatwoot/issue/PLA-121/lock-to-single-thread

---------

Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>

app/views/api/v1/models/_message.json.jbuilder

@@ -12,3 +12,5 @@ json.private message.private
 json.source_id message.source_id
 json.sender message.sender.push_event_data if message.sender
 json.attachments message.attachments.map(&:push_event_data) if message.attachments.present?
+
+json.set! :call, message.call.push_event_data if message.content_type == 'voice_call' && message.respond_to?(:call) && message.call.present?