test(podman): move preflight coverage into driver crate

[elezar]

Summary

Move podman_preflight out of the gateway-backed E2E suite and into the normal openshell-driver-podman test layer. This follows the ownership direction discussed on #3749: the behavior needs neither Podman nor a gateway and should run in the standard workspace test lane without special CI selection.

Related Issue

Related to #3712. No issue required for this localized test ownership refactor; the implementation follows the review of #3749.

Changes

• Extract the Podman ping retry loop into a directly testable helper without changing production retry behavior.
• Add paused-Tokio-time unit coverage for transient recovery, retry exhaustion, elapsed retry budget, and missing-socket diagnostics.
• Retain one crate-level executable smoke test for argument wiring, bounded non-zero exit, and rendered error output.
• Remove the obsolete E2E test target and file; current main has no special mise task, workflow selection, or coverage exception for this target.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests updated: removed the obsolete podman_preflight E2E target
• cargo test -p openshell-driver-podman (233 tests passed)

Checklist

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