[tidbx] add index failed with "Err:[tikv:9001]PD server timeout: start timestamp may fall behind safe point" when simulated pd leader network latency

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
1，run sysbench
2，add index for one of table
3，simulated pd leader network latency 50ms last for 3mins

timeline
[2026-05-13 08:36:49] ###### start adding index
ALTER TABLE `sbtest1` ADD INDEX `index_test_1778632609487`(`c`)
[2026-05-13 08:36:49] ###### wait for ddl job finish
[2026-05-13 09:00:50] ###### ddl job failed
select job_id, db_name, table_name, job_type, create_time, start_time, end_time, state, query from information_schema.ddl_jobs where query = 'ALTER TABLE `sbtest1` ADD INDEX `index_test_1778632609487`(`c`)'
jobId: 363, job type: add index, state: rollback done

### 2. What did you expect to see? (Required)
add index can succeed

### 3. What did you see instead (Required)
add index failed with "Err:[tikv:9001]PD server timeout: start timestamp may fall behind safe point"

2026-05-13 09:00:32
[2026/05/13 01:00:32.081 +00:00] [INFO] [job_worker.go:447] ["finish DDL job"] [keyspaceName=10916992184747791311] [category=ddl] [jobID=363] [conn=1893735984] [job="ID:363, Type:add index, State:rollback done, SchemaState:none, SchemaID:344, TableID:245, RowCount:70001300, ArgLen:1, start time: 2026-05-13 00:36:49.478 +0000 UTC, Err:[tikv:9001]PD server timeout: start timestamp may fall behind safe point, ErrCount:512, SnapshotVersion:466257866857644089, Version: v2, UniqueWarnings:0"]

### 4. What is your TiDB version? (Required)
2026-05-13 07:07:56 | [2026/05/12 23:07:56.505 +00:00] [INFO] [printer.go:73] ["Welcome to TiDB."] [keyspaceName=10916992184747791311] ["Release Version"=CLOUD.202604.0-abfc4e1aa8] [Edition=Enterprise] ["Git Commit Hash"=abfc4e1aa83d7be98d3defe7f2aa03e6fe76da55] ["Git Branch"=HEAD] ["UTC Build Time"="2026-04-16 08:29:59"] [GoVersion=go1.25.8] ["Race Enabled"=false] ["Check Table Before Drop"=false] ["TiDB Component Version"=v26.4.0-abfc4e1aa8] ["Kernel Type"="Next Generation"] ["Enterprise Extension Commit Hash"=dc633aae52eb11b4e3549e3a7ef5b0ed14e159b1]