batch: do not fail migrating batch allocations

[chrisroberts]

Description

Migrating batch allocations being marked as failed when killed was
introduced in dfa07e1 (#26025) to ensure that allocations were
properly rescheduled to another node when drained if no eligible
nodes were immediately available.

It appears that the change introduced in 333dd94 (#26292) resolved
the underlying issue affecting drained batch allocations by filtering
allocations with a desired state that is terminal when reconciling
the task group.

This reverts the changes to mark the migrating batch allocations
as failed. Draining batch nodes still behave as expected and the
status of the migrated allocations are now complete instead of
failed.

Testing & Reproduction steps

Links

Reverts changes introduced in #26025
Related changes that fixed the drain issue #26292

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Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

[tgross]

The code change seems fine but after a little digging the mechanism is a little unclear to me.

I wanted to double-check the expected behavior, and the nomad node drain docs say:

Allocations for batch and sysbatch jobs will wait until they complete or the drain's deadline is reached, whichever comes first. These allocations will not be replaced.

So the original #26025 PR now seems like it's got an issue in terms of what we were trying to fix (I definitely missed this during the original review so that's on me). And the PR you mentioned in the description here #26292 was never backported because it wasn't considered a bug fix, but when I backported the patch in this PR to the release/1.8.x+ent branch for testing, everything still seemed to work as expected, so that couldn't be what's fixing the underlying issue.