feat(ci): notify #team-fairies when backwards-compat e2e fails on nightly (#23006)

Forward-port of
#23003 (merged to
v4-next via the backport-staging branch) onto `merge-train/fairies`, to
keep the v4 ↔ v5 diff small.

## Summary

Same change as #23003: wraps `bootstrap_ec2 "./bootstrap.sh
ci-compat-e2e"` in `ci.sh` so that on a `-nightly.*` tag a failing run
posts to `#team-fairies` via the existing `ci3/slack_notify` helper,
then exits with the captured rc (so GH job result is unchanged). The
compat-e2e job is `continue-on-error: true` on nightlies
(`.github/workflows/ci3.yml`), so failures otherwise go unnoticed.

Cherry-picked cleanly from b9c2250 — no
conflicts.

Closes
[F-632](https://linear.app/aztec-labs/issue/F-632/notify-team-fairies-channel-when-backwards-compat-fails)
on the v5 line.

ClaudeBox log: https://claudebox.work/s/a0aa5c1e0994ac85?run=2

Author: AztecBot

ci.sh

@@ -312,7 +312,15 @@ case "$cmd" in
     export CI_DASHBOARD="releases"
     export JOB_ID="x-compat-e2e"
     export AWS_SHUTDOWN_TIME=300
-    bootstrap_ec2 "./bootstrap.sh ci-compat-e2e"
+    rc=0
+    bootstrap_ec2 "./bootstrap.sh ci-compat-e2e" || rc=$?
+    # On nightly tags compat-e2e is non-blocking (continue-on-error in ci3.yml), so
+    # failures otherwise go unnoticed. Notify #team-fairies so they get picked up.
+    if [ "$rc" -ne 0 ] && [[ "${REF_NAME:-}" == *-nightly.* ]]; then
+      run_url="https://github.com/${GITHUB_REPOSITORY:-AztecProtocol/aztec-packages}/actions/runs/${GITHUB_RUN_ID:-unknown}"
+      "$ci3/slack_notify" "Backwards compatibility e2e tests FAILED on nightly tag <${run_url}|${REF_NAME}>" "#team-fairies"
+    fi
+    exit "$rc"
     ;;
 
   ############