[tidbx] Import into job was still running after target table was dropped

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
1, run import into
2，import into job was running
2, drop target table which was importing

### 2. What did you expect to see? (Required)
the table was importing should not be allowed to be deleted

### 3. What did you see instead (Required)
1，the table was importing was deleted
2，the space of deleted tables can not be reclaimed

### 4. What is your TiDB version? (Required)
v8.5.4-nextgen.202510.24