Backport of scaling policy: use request namespace as target if unset in jobspec into release/1.8.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #24065 to be assessed for backporting due to the inclusion of the label backport/1.8.x.

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

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When jobs are submitted with a scaling policy, the scaling policy's target only includes the job's namespace if the namespace field is set in the jobspec and not from the request. Normally jobs are canonicalized in the RPC handler before being written to Raft. But the scaling policy targets are instead written during the conversion from api.Job to structs.Job. We populate the structs.Job namespace from the request here as well, but only after the conversion has occurred. Swap the order of these operations so that the conversion is always happening with a correct namespace.

Long-term we should not be making mutations during conversion either. But we can't remove it immediately because API requests may come from any agent across upgrades. Move the scaling target creation into the Canonicalize method and mark it for future removal in the API conversion code path.

Fixes: #24039

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Note for reviewers: I've broken this into one commit for the immediate fix and one for the Canonicalize refactor, just in case we think the latter is too much to land post-beta. I think it's safe, but I'm open to arguments otherwise.

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Overview of commits

• 5e1ad14

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