fix(podman): reuse IPv4 primary listener on macOS

[matthewgrossman]

Summary

Remove the stale macOS Podman IPv6 listener split now that the gateway reuses a primary listener that covers the driver callback address. Local Podman workflows return to the standard 127.0.0.1 gateway endpoint.

Related Issue

No issue required: this is a localized correction that removes the workaround introduced in #2622 after primary-listener reuse landed in #2739.

Changes

• Remove the macOS Podman ::1 primary-listener override from the local gateway launcher.
• Exercise the gateway built-in IPv4 loopback default in the Podman E2E harness.
• Remove the IPv6-specific health-probe proxy bypass.
• Preserve driver callback-address discovery so Linux and explicit non-covering binds can still receive a callback-only listener.

Testing

• mise run pre-commit passes
• cargo test -p openshell-server gateway_listener passes (17 tests)
• mise run e2e:podman (running separately in the main task)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)