Preserve pack metadata for jobs with top-level blocks like locals

[gautamg795]

Description
Nomad Pack injects pack.* metadata into rendered job files so later plan, run, stop, and destroy commands can identify jobs managed by the same pack deployment.

The metadata injection path currently parses the rendered file with a schema that only permits top-level job blocks. If a valid Nomad job file also contains another top-level block, such as locals, that parse reports diagnostics and Nomad Pack skips metadata injection. The job can still be submitted, but it loses the pack.* metadata and future Nomad Pack operations treat it as unmanaged.

This changes the metadata injection parser to use PartialContent, so it can find top-level job blocks while preserving unrelated top-level HCL blocks. A regression test covers a job file with top-level locals.

Reminders

• Add CHANGELOG.md entry

Changes to Security Controls

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

No. This does not change access controls, encryption, logging, or other security controls.

[hashicorp-cla-app]

All committers have signed the CLA.