fix: Force account_id in the query (#13388)

### What

Forces `account_id` to be applied consistently in queries and message creation paths.

### Why

Some queries were missing `account_id`, leading to cross-account scans and slow performance in large datasets.

### Changes

* Added `account_id` to the relevant query columns.
* Ensured messages are always created within the correct account scope.
* Updated `created_at` handling where required for consistency.

### Impact

* Prevents cross-account queries.
* Improves query performance.
* Reduces risk of incorrect data access across accounts.

### Notes

No functional behavior change for end users. This is a performance and safety fix.

app/services/conversations/message_window_service.rb

@@ -65,6 +65,6 @@ class Conversations::MessageWindowService
   end
 
   def last_incoming_message
-    @last_incoming_message ||= @conversation.messages&.incoming&.last
+    @last_incoming_message ||= @conversation.messages.where(account_id: @conversation.account_id).incoming&.last
   end
 end