add index can not be canceled timely

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

`func (dc *ddlCtx) handleRangeTasks` is executed on a single goroutine. There's a loop that process 1024 (`backfillTaskChanSize`) tasks. And each task has a network RPC to adjust end key

https://github.com/pingcap/tidb/blob/d7ce2f2faa1da3177a0f0a7e825f6e8fccd13ec8/ddl/backfilling.go#L505-L511

In the log, it costs 6 seconds to finish one loop

```
[2024/10/12 20:22:01.257 +08:00] [INFO] [backfilling.go:511] ["[ddl] send range task to workers, change end key"] ...
...
[2024/10/12 20:22:06.922 +08:00] [INFO] [backfilling.go:511] ["[ddl] send range task to workers, change end key"] ...
```

So 6 second * 1024 = 1.7h is needed to finish `handleRangeTasks`, then the job cancelation can be seen

### 2. What did you expect to see? (Required)

### 3. What did you see instead (Required)

### 4. What is your TiDB version? (Required)

this is an ONCALL, the version I checked is v6.5.4

Start in ddl/backfilling.go at ddlCtx.handleRangeTasks and the referenced loop around lines 505–511. Reproduce the cancellation scenario described in the issue and trace how the 1024-task loop and network RPCs respond to cancellation. Done means cancellation is observed without waiting for the full loop to finish.

Written by the indexing model from the issue text.