Add slack_user to firewatch webhook test rule

Wire up per-rule Slack routing in the test config so pj-rehearse
validates that the webhook payload includes the slack_user field.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: amp-rh

ci-operator/config/RedHatQE/firewatch/RedHatQE-firewatch-main__slack-webhook-test.yaml

@@ -29,7 +29,7 @@ tests:
       FIREWATCH_CONFIG: |
         {
           "failure_rules": [
-            {"step": "*", "failure_type": "all", "classification": "Test", "jira_project": "INTEROP"}
+            {"step": "*", "failure_type": "all", "classification": "Test", "jira_project": "INTEROP", "slack_user": "mpruitt@redhat.com"}
           ]
         }
       FIREWATCH_DEFAULT_JIRA_PROJECT: INTEROP