Why switch from external to internal polling is not allowed?

when change config from:
```c
ssl_engine {
use_engine qatengine;
default_algorithms RSA,EC,DH,DSA;
qat_engine {
qat_offload_mode async;
qat_notify_mode poll;

qat_poll_mode external;
qat_external_poll_interval 1;
qat_shutting_down_release on;
}
}
```

to:
```c
ssl_engine {
use_engine qatengine;
default_algorithms RSA,EC,DH,DSA;
qat_engine {
qat_offload_mode async;
qat_notify_mode poll;
}
}
```
nginx write emerg log like:
[emerg] 248#0: Switch from external to internal polling is invalid, and still use external polling

Why switch from external to internal polling is not allowed?

Start by reproducing the configuration change from external polling to internal polling and confirm the reported emerg log. Trace the QAT polling configuration handling to determine whether this transition is intentionally rejected or mishandled; done means documenting the reason or providing a verified fix with regression coverage.

Written by the indexing model from the issue text.