fix: Add account_id to the query to force using a better index (#12420)

I've added the account_id filter to the
`get_agent_ids_over_assignment_limit` method. This optimization will
help the query leverage the existing composite index
`conv_acid_inbid_stat_asgnid_idx (account_id, inbox_id, status,
assignee_id)` for better performance.

**Before:**
```sql
HashAggregate (cost=224238.12..224256.27 rows=484 width=4)
Group Key: assignee_id
Filter: (count(*) >= 10)
-> Index Scan using index_conversations_on_inbox_id on conversations (cost=0.44..223963.67 rows=54891 width=4)
Index Cond: (inbox_id = ???)
Filter: (status = 0)
```

**After:**
```sql
GroupAggregate (cost=0.44..5688.30 rows=476 width=4)
Group Key: assignee_id
Filter: (count(*) >= 10)
-> Index Only Scan using conv_acid_inbid_stat_asgnid_idx on conversations (cost=0.44..5640.81 rows=5928 width=4)
Index Cond: ((account_id = ??) AND (inbox_id = ??) AND (status = 0))
```

enterprise/app/models/enterprise/inbox.rb

@@ -22,7 +22,13 @@ module Enterprise::Inbox
   end
 
   def get_agent_ids_over_assignment_limit(limit)
-    conversations.open.select(:assignee_id).group(:assignee_id).having("count(*) >= #{limit.to_i}").filter_map(&:assignee_id)
+    conversations
+      .open
+      .where(account_id: account_id)
+      .select(:assignee_id)
+      .group(:assignee_id)
+      .having("count(*) >= #{limit.to_i}")
+      .filter_map(&:assignee_id)
   end
 
   def ensure_valid_max_assignment_limit