IMPORT INTO + global sort didn't handle UK conflict correctly

[lance6716]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

diff --git a/tests/realtikvtest/importintotest4/global_sort_test.go b/tests/realtikvtest/importintotest4/global_sort_test.go
index 6734ff65ee..3563ad5c89 100644
--- a/tests/realtikvtest/importintotest4/global_sort_test.go
+++ b/tests/realtikvtest/importintotest4/global_sort_test.go
@@ -156,3 +156,28 @@ func (s *mockGCSSuite) TestGlobalSortMultiFiles() {
 	s.tk.MustQuery(importSQL)
 	s.tk.MustQuery("select * from t").Sort().Check(testkit.Rows(allData...))
 }
+
+func (s *mockGCSSuite) TestGlobalSortUniqueKeyConflict() {
+	var allData []string
+	for i := 0; i < 10; i++ {
+		var content []byte
+		keyCnt := 1000
+		for j := 0; j < keyCnt; j++ {
+			idx := i*keyCnt + j
+			content = append(content, []byte(fmt.Sprintf("%d,test-%d\n", idx, j))...)
+		}
+		s.server.CreateObject(fakestorage.Object{
+			ObjectAttrs: fakestorage.ObjectAttrs{BucketName: "gs-multi-files-uk", Name: fmt.Sprintf("t.%d.csv", i)},
+			Content:     content,
+		})
+	}
+	slices.Sort(allData)
+	s.prepareAndUseDB("gs_multi_files")
+	s.server.CreateBucketWithOpts(fakestorage.CreateBucketOpts{Name: "sorted"})
+	s.tk.MustExec("create table t (a bigint primary key , b varchar(100) unique key);")
+	// 1 subtask, encoding 10 files using 4 threads.
+	sortStorageURI := fmt.Sprintf("gs://sorted/gs_multi_files?endpoint=%s", gcsEndpoint)
+	importSQL := fmt.Sprintf(`import into t FROM 'gs://gs-multi-files-uk/t.*.csv?endpoint=%s'
+		with cloud_storage_uri='%s', __max_engine_size='1', thread=8`, gcsEndpoint, sortStorageURI)
+	s.tk.MustQuery(importSQL)
+}

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

has reasonable output

3. What did you see instead (Required)

in the log we can find

[2025/02/19 20:55:53.969 +08:00] [WARN] [local.go:1255] ["meet retryable error when writing to TiKV"] [category=ddl-ingest] [error="peer 31, store 1, region 30, epoch conf_ver:1 version:11 , when send data: rpc error: code = Unknown desc = EngineTraits(Engine(Status { code: IoError, sub_code: None, sev: NoError, state: \"Invalid argument: Keys must be added in strict ascending order.\" }))"] ["job stage"=needRescan]

4. What is your TiDB version? (Required)

[lance6716]

should also port #59611 to master

And check if DDL has this problem

[D3Hunter]

Ensure that the data to be imported does not contain any records with primary key or non-null unique index conflicts. Otherwise, the conflicts can result in import task failures.

https://docs.pingcap.com/tidb/dev/sql-statement-import-into

by design, we don't support conflict data, but we should fail with a more clear error in this case