fix(sandbox): resolve host-gateway aliases from proxy /etc/hosts

[zanetworker]

Summary

• Fix SSRF engine blocking host.openshell.internal in 0.0.91 by adding a DNS resolution fallback that reads the proxy's own /etc/hosts for host-gateway aliases.
• The sandbox process runs in a separate mount namespace whose /etc/hosts does not contain the driver-injected entry, and the system DNS resolver cannot resolve proxy-internal names. The proxy's /etc/hosts (populated by the compute driver's --add-host flag) is the correct source of truth.

Related Issue

Fixes #2478

Changes

crates/openshell-supervisor-network/src/proxy.rs

• Added resolve_from_proxy_hosts() (async) backed by a path-parameterized resolve_from_hosts_file() helper — reads the proxy's own /etc/hosts via tokio::fs (matches the sibling resolve_from_sandbox_hosts I/O pattern) using the existing parse_hosts_file_for_host parser. Scoped to Linux + test. Read failures log at debug! and return None (fall through to DNS) rather than failing silently.
• Extended resolve_socket_addrs() with a new resolution step between sandbox /etc/hosts and system DNS: for host-gateway aliases only, try the proxy's /etc/hosts as a fallback.
• Added 3 unit tests against a temp hosts file exercising the real file-reading path: alias resolves, missing alias → None, unreadable file → None.

Resolution order is now:

1. IP literal
2. Sandbox /etc/hosts (via /proc/{pid}/root/etc/hosts)
3. Proxy /etc/hosts (host-gateway aliases only) ← new
4. System DNS

Security properties preserved:

• SSRF guards unchanged (link-local-only for trusted-gateway exemption)
• Resolved IPs still flow through full SSRF validation (allowed_ips or exact_declared_endpoint_host path)
• Cloud metadata, loopback, and control-plane ports still always blocked
• Fallback only triggers for 3 hardcoded aliases in HOST_GATEWAY_ALIASES

Testing

• 1011 unit tests pass (cargo test -p openshell-supervisor-network)
• Clippy clean (cargo clippy --workspace --all-targets -- -D warnings)
• Format clean (cargo fmt --all -- --check)
• License headers pass
• E2E sandbox_reaches_host_openshell_internal_via_host_gateway_alias — PASS (podman, macOS arm64, applehv, bridge network gateway_ip=10.89.0.1)
• E2E sandbox_inference_local_routes_to_host_openshell_internal — PASS
• E2E inference_set_supports_no_verify_for_unreachable_endpoint — PASS
• Manual verification: sandbox with allowed_ips policy reached host server; sandbox without policy correctly denied; non-alias hostname correctly denied

Checklist

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

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