Fail to add unique key using global sort in large data

[CbcWestwolf]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Add unique key using global sort in large data

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

Succeed

3. What did you see instead (Required)

[2025/02/18 10:05:47.292 +00:00] [ERROR] [task_executor.go:344] ["run subtask failed"] [task-id=30001] [task-type=backfill] [error="insufficient bytes to decode value"] [errorVerbose="insufficient bytes to decode value
github.com/pingcap/tidb/pkg/lightning/common.DupDetectKeyAdapter.Decode
    /workspace/source/tidb/pkg/lightning/common/key_adapter.go:91
github.com/pingcap/tidb/pkg/lightning/backend/external.(*Engine).GetRegionSplitKeys
    /workspace/source/tidb/pkg/lightning/backend/external/engine.go:470
github.com/pingcap/tidb/pkg/lightning/backend/local.getRegionSplitKeys
    /workspace/source/tidb/pkg/lightning/backend/local/local.go:885
github.com/pingcap/tidb/pkg/lightning/backend/local.(*Backend).ImportEngine
    /workspace/source/tidb/pkg/lightning/backend/local/local.go:1327
github.com/pingcap/tidb/pkg/ddl.(*cloudImportExecutor).RunSubtask
    /workspace/source/tidb/pkg/ddl/backfilling_import_cloud.go:145
github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*BaseTaskExecutor).runSubtask.func1
    /workspace/source/tidb/pkg/disttask/framework/taskexecutor/task_executor.go:460
github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*BaseTaskExecutor).runSubtask
    /workspace/source/tidb/pkg/disttask/framework/taskexecutor/task_executor.go:461
github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*BaseTaskExecutor).Run
    /workspace/source/tidb/pkg/disttask/framework/taskexecutor/task_executor.go:339
github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*Manager).startTaskExecutor.func2
    /workspace/source/tidb/pkg/disttask/framework/taskexecutor/manager.go:377
github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).RunWithLog.func1
    /workspace/source/tidb/pkg/util/wait_group_wrapper.go:181
runtime.goexit
    /root/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.23.6.linux-amd64/src/runtime/asm_amd64.s:1700"]

4. What is your TiDB version? (Required)

master