feat(server): disable IPv6 in execd init for k8s egress; refactor helpers

[Pangjiping]

Summary

• When networkPolicy and egress image are set, privileged execd-installer writes /proc/sys/net/ipv6/conf/all/disable_ipv6 in the pod netns; egress sidecar only needs CAP_NET_ADMIN (no privileged, no sysctl binary)
• Remove separate egress-disable-ipv6 init container
• Move V1SecurityContext dict conversion to k8s/security_context.py
• egress_helper: prep_execd_init_for_egress, apply_egress_to_spec, build_security_context_for_sandbox_container only; drop unused pod_spec from apply_egress_to_spec
• Fix test_kubernetes_service async tests missing await on create_sandbox

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

[ninan-nn]

LGTM