fix: Prevent creating duplicate messages via Instagram echo events (#11535)

Fixes
https://linear.app/chatwoot/issue/CW-4383/copilot-resolution-message-creating-duplicate-conversations-due-to
and
https://linear.app/chatwoot/issue/CW-4287/instagram-channel-missing-existing-source-id-check

When the copilot/system resolves a conversation with a system resolve
message (with `lock_to_single_conversation` disabled), echo events were
creating new conversations instead of using existing ones. This occurred
because we were checking the echo_id in the new conversation rather than
in the resolved conversation. This PR fixes the issue by checking if the
message exists anywhere in the system instead of checking within a
particular conversation.

app/builders/messages/instagram/base_message_builder.rb

@@ -152,11 +152,13 @@ class Messages::Instagram::BaseMessageBuilder < Messages::Messenger::MessageBuil
   end
 
   def message_already_exists?
-    cw_message = conversation.messages.where(
-      source_id: @messaging[:message][:mid]
-    ).first
+    find_message_by_source_id(@messaging[:message][:mid]).present?
+  end
 
-    cw_message.present?
+  def find_message_by_source_id(source_id)
+    return unless source_id
+
+    @message = Message.find_by(source_id: source_id)
   end
 
   def all_unsupported_files?