Backport of client: 404 when accessing files for GC'ed alloc into release/1.4.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #18232 to be assessed for backporting due to the inclusion of the label backport/1.4.x.

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

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When an allocation is garbage collected from the client, but not from the servers, the API request is routed to the client and the client does attempt to read the file, but the alloc files have already been deleted, resulting in a 500 error.

This happens because the client GC only destroys the alloc runner (deleting the alloc dir), but it keeps a reference to the alloc runner until the alloc is garbage collected from the servers as well.

This commit adjusts this logic by checking if the alloc runner (and the alloc files) has been destroyed, returning a 404 if so.

Closes #18216

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Overview of commits

• 14a38be