When `_stop?wait_for_checkpoint=false` and
`_stop?force=true&wait_for_checkpoint=false` are called, there is a
small chance that the Transform Indexer thread will run if it is
scheduled before the stop API is called but before the threadpool runs
the executable. The `onStart` method now checks the state of the
indexer before executing. This will mitigate errors caused by reading
from Transform internal indices while the Task is stopped or deleted.

This does not impact when `wait_for_checkpoint=true`, because the
indexer state will remain `INDEXING` until the checkpoint is finished.

Relate elastic#107266