fix(aci): Make associate_new_group_with_detector more robust (#103418)

Since we know Detectors can go away (even before we have a chance to
process a new occurrence) and we have a convention for representing that
with DetectorGroup, update associate_new_group_with_detector
accordingly.

Author: kcons

src/sentry/workflow_engine/processors/detector.py

@@ -382,6 +382,28 @@ def associate_new_group_with_detector(group: Group, detector_id: int | None = No
                 },
             )
             return False
+
+    # Check if the detector exists. If not, create DetectorGroup with null detector_id
+    # to make it clear that we were associated with a detector that no longer exists.
+    if not Detector.objects.filter(id=detector_id).exists():
+        metrics.incr(
+            "workflow_engine.associate_new_group_with_detector",
+            tags={"group_type": group.type, "result": "detector_missing"},
+        )
+        logger.warning(
+            "associate_new_group_with_detector_detector_missing",
+            extra={
+                "group_id": group.id,
+                "group_type": group.type,
+                "detector_id": detector_id,
+            },
+        )
+        DetectorGroup.objects.get_or_create(
+            detector_id=None,
+            group_id=group.id,
+        )
+        return True
+
     DetectorGroup.objects.get_or_create(
         detector_id=detector_id,
         group_id=group.id,

tests/sentry/workflow_engine/processors/test_detector.py

@@ -1058,3 +1058,15 @@ def test_feedback_group_returns_false(self) -> None:
         group = self.create_group(project=self.project, type=FeedbackGroup.type_id)
         assert not associate_new_group_with_detector(group)
         assert not DetectorGroup.objects.filter(group_id=group.id).exists()
+
+    def test_deleted_detector_creates_null_association(self) -> None:
+        group = self.create_group(project=self.project, type=MetricIssue.type_id)
+        deleted_detector_id = self.metric_detector.id
+
+        self.metric_detector.delete()
+
+        assert associate_new_group_with_detector(group, deleted_detector_id)
+
+        detector_group = DetectorGroup.objects.get(group_id=group.id)
+        assert detector_group.detector_id is None
+        assert detector_group.group_id == group.id