test(podman): harden preflight retry coverage

[elezar]

Summary

Stacked follow-up to #3783. That PR now moves podman_preflight into the normal openshell-driver-podman integration-test layer and makes its diagnostic assertion portable on macOS. This PR adds deterministic unit coverage for the driver's retry contract and reduces the executable coverage to one smoke test.

This PR depends on #3783. Because #3783 originates from a fork, GitHub cannot use its head branch as this PR's base; #3783 head b9c4f2cf7 is nevertheless the direct parent of this PR's commit. Once #3783 merges, the displayed diff will collapse to only the retry-coverage follow-up.

Related Issue

Related to #3712 and follows the ownership direction discussed on #3749.

Changes

• Extract the production ping retry loop into a directly testable helper without changing retry behavior.
• Add paused-Tokio-time tests for transient recovery, retry exhaustion, exact attempt count, and elapsed retry budget.
• Cover the concrete missing-socket connection error at the client boundary.
• Consolidate the two executable tests from test(podman): move podman_preflight into driver-podman integration tests #3783 into one smoke test for argument wiring, bounded non-zero exit, and rendered diagnostics.

Testing

• mise run pre-commit passes
• cargo test -p openshell-driver-podman — 233 tests passed
• Unit tests added/updated
• Integration test updated
• mise run test — blocked by an unrelated existing tasks/scripts/test-install-sh.sh command-substitution syntax failure in linux_package_method

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; test coverage only)

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.