fix(autofix): Handle Group.DoesNotExist and None group gracefully

- Add try-except for Group.DoesNotExist in execute() to handle cases where
  group is deleted between autofix job start and completion webhook
- Add None check in _maybe_trigger_supergroups_embedding() to handle edge
  case where group parameter is None
- Both fixes ensure graceful degradation instead of unhandled exceptions

Co-authored-by: Armen Zambrano G. <armenzg@users.noreply.github.com>

Author: cursoragent

src/sentry/seer/autofix/on_completion_hook.py

@@ -82,7 +82,15 @@ def execute(cls, organization: Organization, run_id: int) -> None:
         if group_id is None:
             return
 
-        group = Group.objects.get(id=group_id, project__organization_id=organization.id)
+        try:
+            group = Group.objects.get(id=group_id, project__organization_id=organization.id)
+        except Group.DoesNotExist:
+            logger.info(
+                "autofix.on_completion_hook.group_not_found",
+                extra={"run_id": run_id, "organization_id": organization.id, "group_id": group_id},
+            )
+            return
+
         group.update(seer_explorer_autofix_last_triggered=timezone.now())
 
         # Send webhook for the completed step
@@ -227,13 +235,16 @@ def _maybe_trigger_supergroups_embedding(
         organization: Organization,
         run_id: int,
         state: SeerRunState,
-        group: Group,
+        group: Group | None,
     ) -> None:
         """Trigger supergroups embedding if feature flag is enabled."""
         current_step = cls._get_current_step(state)
         if current_step != AutofixStep.ROOT_CAUSE:
             return
 
+        if group is None:
+            return
+
         if not features.has("projects:supergroup-embeddings-explorer", group.project):
             return