fix(aci milestone 3): fix priority fetching (#98981)

Priority is a field on the occurrence, not stored in the evidence data.
:oops:

Author: mifu67

src/sentry/workflow_engine/models/incident_groupopenperiod.py

@@ -132,7 +132,9 @@ def create_incident_for_open_period(cls, occurrence, alert_rule, group, open_per
         # XXX: if this is the very first open period, or if the priority didn't change from the last priority on the last open period,
         # manually add the first incident status change activity because the group never changed priority
         # if the priority changed, then the call to update_incident_status in update_priority will be a no-op.
-        priority = occurrence.evidence_data.get("priority", DetectorPriorityLevel.HIGH)
+        priority = (
+            occurrence.priority if occurrence.priority is not None else DetectorPriorityLevel.HIGH
+        )
         severity = (
             IncidentStatus.CRITICAL
             if priority == DetectorPriorityLevel.HIGH