fix: optimize message query with account_id filter (#13759)

## Description

This PR optimizes message queries by explicitly filtering with
`account_id` so the database can use the existing indexes more
efficiently.

Changes:
- Add `account_id` to message query filters to improve index
utilization.
- Update `last_incoming_message` query to include `account_id`.
- Avoid unnecessary preloading of `contact_inboxes` where it is not
required.
- Update specs to ensure `account_id` is set correctly in
message-related tests.

These changes reduce query cost and improve performance for message
lookups, especially on large accounts.

---------

Co-authored-by: Pranav <pranav@chatwoot.com>

app/models/conversation.rb

@@ -143,7 +143,7 @@ class Conversation < ApplicationRecord
   end
 
   def last_incoming_message
-    messages&.incoming&.last
+    messages.where(account_id: account_id)&.incoming&.last
   end
 
   def toggle_status