v3cli controller's pexpect session destoryed by timeout

## Summary

This issue increases worker API’s response time about 4 seconds.

## Logs

```
2025-10-24 11:11:46.136 WARNING ai.backend.v3_worker.v3net_controller [3893217] Command failed on attempt 1: End Of File (EOF). Exception style platform.

command: /usr/local/v3net/bin/v3cli
args: [b'/usr/local/v3net/bin/v3cli']
buffer (last 100 chars): ''
before (last 100 chars): ' timeout\r\r\naction scan start directory /home/lablup/backend.ai/volumes\r\n'
after:
match: None
match_index: None
exitstatus: 0
flag_eof: True
pid: 3893223
child_fd: 30
closed: False
timeout: None
delimiter:
logfile: None
logfile_read: None
logfile_send: None
maxread: 2000
ignorecase: False
searchwindowsize: None
delaybeforesend: 0.05
delayafterclose: 0.1
delayafterterminate: 0.1
searcher: searcher_re:
0: re.compile('v3net>')
```

JIRA Issue: BA-2805