import into failed with “syntax error: unterminated quoted field” when simulate network partition between ddl owner and pd leader

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
1、run sysbench
2、import into with global sort
3、simulate network partition between ddl owner and pd leader last for 3mins

### 2. What did you expect to see? (Required)
import into can succeed

### 3. What did you see instead (Required)
import into failed when simulate network partition between ddl owner and pd leader
chaos: 2025/09/26 00:23:55.179 +08:00 ~ 2025/09/26 00:26:55.180 +08:00

`[2025/09/26 00:26:45.494 +08:00] [ERROR] [parser.go:248] ["syntax error"] [pos=68157437] [content=\ufffdp]
[2025/09/26 00:26:45.494 +08:00] [ERROR] [chunk_process.go:334] ["process chunk failed"] [type=ImportInto] [table-id=188] [key=brie/lightning-mhy-100G-csv/sysbench.user_data1.000001243.csv:0] [takeTime=6.997477892s] [error="[Lightning:Restore:ErrEncodeKV]encode kv error in file brie/lightning-mhy-100G-csv/sysbench.user_data1.000001243.csv:0 at offset 68157440: syntax error: unterminated quoted field"]
[2025/09/26 00:26:45.494 +08:00] [INFO] [subtask_executor.go:56] ["execute chunk"] [type=ImportInto] [table-id=188]
[2025/09/26 00:26:45.509 +08:00] [WARN] [s3.go:935] ["read s3 object failed, will retry"] [file=brie/lightning-mhy-100G-csv/sysbench.user_data1.000001252.csv] [retryCnt=1] [error="unexpected EOF"]
[2025/09/26 00:26:45.651 +08:00] [INFO] [chunk_process.go:330] ["process chunk start"] [type=ImportInto] [table-id=188] [key=brie/lightning-mhy-100G-csv/sysbench.user_data1.000001253.csv:0]
[2025/09/26 00:26:45.789 +08:00] [ERROR] [parser.go:248] ["syntax error"] [pos=178257860] [content="\ufffd\ufffd&\ufffd\ufffdl;\ufffdк\ufffd\u000e\ufffd\ufffdz\u0003\ufffd\ufffdi؛\ufffdM\ufffd\u0004͊u\ufffd\u001b\u000cL\ufffd\ufffd\ufffd0\ufffd\ufffd\ufffdG\ufffdj#\ufffd0 \ufffd\u000b\ufffd\ufffd\u000eO\u000f\t\ufffdt\ufffd\u0001/"]
[2025/09/26 00:26:45.789 +08:00] [ERROR] [chunk_process.go:334] ["process chunk failed"] [type=ImportInto] [table-id=188] [key=brie/lightning-mhy-100G-csv/sysbench.user_data1.000001236.csv:0] [takeTime=10.896565951s] [error="[Lightning:Restore:ErrEncodeKV]encode kv error in file brie/lightning-mhy-100G-csv/sysbench.user_data1.000001236.csv:0 at offset 178257920: syntax error: unterminated quoted field"]
[2025/09/26 00:26:45.790 +08:00] [WARN] [s3.go:935] ["read s3 object failed, will retry"] [file=brie/lightning-mhy-100G-csv/sysbench.user_data1.000001245.csv] [retryCnt=2] [error="unexpected EOF"]
[2025/09/26 00:26:45.790 +08:00] [ERROR] [encode_and_sort_operator.go:108] ["error on encode and sort"] [type=ImportInto] [task-id=90007] [step=encode&sort] [error="[Lightning:Restore:ErrEncodeKV]encode kv error in file brie/lightning-mhy-100G-csv/sysbench.user_data1.000001236.csv:0 at offset 178257920: syntax error: unterminated quoted field"]
[2025/09/26 00:26:45.804 +08:00] [ERROR] [parser.go:248] ["syntax error"] [pos=120586205] [content="X\ufffdv\ufffd\ufffd;iO\ufffdq}S\u0006S0{\u0007\u001a\u000bRC\u000e\ufffdx\ufffd\ufffd\ufffd\ufffd\u0016\ufffdh\ufffd\ufffd"]
[2025/09/26 00:26:45.804 +08:00] [ERROR] [chunk_process.go:334] ["process chunk failed"] [type=ImportInto] [table-id=188] [key=brie/lightning-mhy-100G-csv/sysbench.user_data1.000001247.csv:0] [takeTime=6.695392821s] [error="[Lightning:Restore:ErrEncodeKV]encode kv error in file brie/lightning-mhy-100G-csv/sysbench.user_data1.000001247.csv:0 at offset 120586240: syntax error: unterminated quoted field"]
[2025/09/26 00:26:45.804 +08:00] [ERROR] [encode_and_sort_operator.go:108] ["error on encode and sort"] [type=ImportInto] [task-id=90007] [step=encode&sort] [error="[Lightning:Restore:ErrEncodeKV]encode kv error in file brie/lightning-mhy-100G-csv/sysbench.user_data1.000001247.csv:0 at offset 120586240: syntax error: unterminated quoted field"]`

### 4. What is your TiDB version? (Required)
./tidb-server -V
Release Version: v8.5.4
Edition: Community
Git Commit Hash: 10c40673b9a2c9da953c12cf2b0cf586459096ac
Git Branch: HEAD
UTC Build Time: 2025-09-25 00:23:40
GoVersion: go1.23.8
Race Enabled: false
Check Table Before Drop: false
Store: unistore