fix(gateway): handle delayed Podman bridge listeners

[elezar]

Summary

Fix rootful Podman gateway startup when Podman has allocated a managed bridge gateway address but netavark has not assigned that address yet. The gateway uses delayed exact binding where Linux supports it and a narrowly scoped fallback for the nested Fedora canary topology.

This PR is stacked on #3184, which adds the reusable rootful Podman test-guest configuration.

Related Issue

No issue required: this is a focused fix for a reliably failing Fedora release-canary regression. Longer-term driver-owned callback relay work remains tracked in #2540.

Release Canary reference: https://github.com/NVIDIA/OpenShell/actions/runs/32448768823/job/96673135310

Changes

• Add an additive exact-bind listener requirement that allows trusted built-in drivers to identify addresses eligible for delayed binding while preserving the legacy protobuf field.
• Allow only the built-in rootful Podman driver, using its automatically discovered managed-bridge address, to request delayed binding.
• Use Linux IP_FREEBIND when the bridge gateway address has not yet been assigned.
• When delayed exact binding is unavailable in the nested Linux container topology, use an IPv4 wildcard socket with primary scope only for loopback destinations and callback-only scope elsewhere.
• Document the listener behavior and Podman networking constraints.
• Run Fedora conformance against candidate RPMs in rootless and rootful Podman modes with smoke and gateway-restart scenarios.

Security impact

The nested-container fallback exposes only the sandbox-callable gRPC surface on non-loopback IPv4 destinations in the outer container namespace. User, administrator, health, reflection, non-callback inference, and HTTP routes remain unavailable there. Sandbox authentication and the surrounding container-network boundary remain required defenses.

Testing

• mise run pre-commit
• cargo check -p openshell-server -p openshell-driver-podman
• Targeted Podman and Docker listener/callback tests available on macOS
• git diff --check origin/main..HEAD
• Fedora rootless and rootful conformance lanes (GitHub Actions)

Checklist

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

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[github-actions]

Label test:e2e applied, but pull-request/2874 does not exist yet. A maintainer needs to comment /ok to test e2e39b416207323bc6ef34480f6134efc0ddf2f1 to mirror this PR. Once the mirror exists, re-apply the label or re-run Branch E2E Checks from the Actions tab.

[elezar]

/ok-to-test e2e39b4

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-2874.docs.buildwithfern.com/openshell

[elezar]

/ok-to-test fa02b7d

[elezar]

/ok-to-test 1ca8c32

[elezar]

/ok-to-test 5c8c586