Backport of client: ensure task only runs with prestart hooks into release/1.5.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #18662 to be assessed for backporting due to the inclusion of the label backport/1.5.x.

The below text is copied from the body of the original PR.

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Since the allocation in the task runner is updated in a separate goroutine, a race condition may happen where the task is started but the prestart hooks are skipped because the allocation became terminal.

Checking for a terminal allocation before proceeding with the task start ensures the task only runs if the prestart hooks are also executed.

Since shouldShutdown() only uses terminal allocation status, it remains true after the first transition, so it's safe to check it again after the prestart hooks as it will never revert to false.

Some other implementations ideas I considered:

1. Move the check for shouldShutdown() from within prestart() to before it is called. I think this would be more or less equivalent to this approach.
2. Create a read lock on the task runner alloc so that shouldShutdown() is guaranteed not to change while prestart() runs and the task starts. This is probably the most "correct" approach, but since shouldShutdown() can only transition from false to true, checking it again after prestart() seems enough.

Closes #18659

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Overview of commits

• ed204e0