fix(ci): prevent alert delivery errors from masking test results

Add continue-on-error to the incident.io alert step so that delivery
failures (e.g. "no alerts to resolve" on success runs, transient 5xx,
rate limits) don't mark the job as failed when tests actually passed.

Author: dobrac

.github/workflows/periodic-test.yml

@@ -105,6 +105,7 @@ jobs:
 
       - name: Send incident.io alert
         if: always()
+        continue-on-error: true
         env:
           JOB_STATUS: ${{ job.status }}
           DOMAIN: ${{ matrix.e2b-domain }}