fix(test): retry transparent gateway egress check (daily canary flake, curl exit 55) #120

The daily canary failed once after PR #118 was merged: transparent egress curl in tests/integration/verify_gateway.sh died with exit 55 (failed receiving network data) while the gateway container itself was bootstrapped and all netfilter checks passed. A manual re-run of the same image passed, so this is a transient external flake, not a product bug. Harden the check with up to 4 attempts like the other canary egress checks.