feat: Add unified Call model for voice calling (#14026)

Adds a Call model to track voice call state across providers (Twilio,
WhatsApp). This replaces storing call data in
conversation.additional_attributes and provides a foundation for call
analytics multi-call-per-conversation support, and future voice
providers.

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

app/models/message.rb

@@ -450,3 +450,4 @@ class Message < ApplicationRecord
 end
 
 Message.prepend_mod_with('Message')
+Message.include_mod_with('Concerns::Message')