test(engine-manager): make managed-image reclaim test explicit on macOS

[kvnloo]

Description

Downstream first slice for NVIDIA#17.

procImage intentionally returns an empty executable path on macOS because the current implementation relies on Linux /proc. The production stop path therefore declines reclamation when ownership cannot be verified.

TestUninstallTerminatesRunningInstance currently asserts successful managed-image reclamation on every platform, which makes the services test baseline fail on Darwin even though the production decline is documented behavior.

This patch skips only that Linux reclaim contract on Darwin. It does not weaken the ownership check and does not introduce a new executable-resolution primitive.

Release intent

Changelog title

n/a

Changelog body

n/a

Bumps

• services: none
• nvpair-cluster-manager: none
• nvpair-engine-manager: none
• nvpair-errors: none
• nvpair-job-scheduler: none
• nvpair-manual-nodes: none
• nvpair-node-info: none
• nvpair-node-scanner: none
• nvpair-node-settings: none
• nvpair-proxy: none
• nvpair-tui: none
• nvpair-ui-broker: none
• nvpair-workload-manager: none

Scope

In scope:

• make the reclaim regression explicit about its Linux-only executable-resolution contract.

Out of scope:

• changing production reclaim behavior on macOS;
• adding a new executable-resolution primitive.

Validation

Fork CI:

• Build: passed
• CI: passed
• Release-intent validation: fixing the missing required intent block in this PR description

Risk

Test-only change. Production behavior is unchanged.

All commits are DCO-signed and the branch is based on upstream develop @ 937cf6c.