test(proxy-init): enable strict mode in enforce-drop harness

Address CodeRabbit on #484: switch the test harness from set -u to
set -euo pipefail (the repo's shell convention) so command/pipe
failures in the setup phase surface instead of being masked. The
run-all-assertions design is unaffected — asserts return 0 via
if/else and expected-nonzero commands are guarded with || true;
verified all 13 assertions still run and the harness exits 0.

Assisted-By: Claude (Anthropic AI) <noreply@anthropic.com>
Signed-off-by: Hai Huang <huang195@gmail.com>

Author: huang195

authbridge/proxy-init/test-enforce-drop.sh

@@ -16,7 +16,7 @@
 # bash, the dummy kernel module. Runs on Linux / CI (e.g. ubuntu-latest); not
 # on macOS. Uses `unshare --net` (not named `ip netns`) so it also works inside
 # nested containers. Exit code 0 = all pass.
-set -u
+set -euo pipefail
 
 SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
 INIT="${INIT_SCRIPT:-${SCRIPT_DIR}/init-iptables.sh}"