global sort merge-sort/write&ingest not limited by thread

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
Import Into data using global sort with thread=16 vs thread=8

### 2. What did you expect to see? (Required)
Resource usage is limited by thread
### 3. What did you see instead (Required)
You can see that the resource utilization is the same for thread=8 and thread=16.

### 4. What is your TiDB version? (Required)
```sql
[11:18:12]TiDB root:test> select tidb_version();
+-----------------------------------------------------------+
| tidb_version() |
+-----------------------------------------------------------+
| Release Version: v7.6.0-alpha |
| Edition: Community |
| Git Commit Hash: f0a4104fb03ee09c086d7a12144abee8d502117a |
| Git Branch: heads/refs/tags/v7.6.0-alpha |
| UTC Build Time: 2024-01-08 14:33:09 |
| GoVersion: go1.21.5 |
| Race Enabled: false |
| Check Table Before Drop: false |
| Store: tikv |
+-----------------------------------------------------------+
1 row in set
Time: 0.843s
```

Reproduce the global-sort import with thread=8 and thread=16, then trace the merge-sort/write&ingest path responsible for resource usage. Done means resource utilization is constrained by the configured thread count and the two runs no longer use the same resources.

Written by the indexing model from the issue text.