semgrep: Enforce no bare select_related() calls.

Previously, #26419 addressed the majority of these calls, but did not
prevent more from creeping in.  Remove the one remaining
callsite (after the cleanup from the previous commits), and ban any
future use of the pattern.

tools/semgrep-py.yml

@@ -43,6 +43,15 @@ rules:
       exclude:
         - "**/migrations/"
 
+  - id: dont-use-empty-select_related
+    pattern-either:
+      - pattern: $X.objects. ... .select_related()
+      - pattern: $X.objects. ... .prefetch_related()
+    message: |
+      Do not use a bare '.select_related()' or '.prefetch_related()', which can join many more tables than expected.  Specify the relations to follow explicitly.
+    languages: [python]
+    severity: ERROR
+
   - id: dont-import-models-in-migrations
     patterns:
       - pattern-not: from zerver.lib.redis_utils import get_redis_client

zerver/migrations/0273_migrate_old_bot_messages.py

@@ -38,9 +38,7 @@ def fix_messages(apps: StateApps, schema_editor: BaseDatabaseSchemaEditor) -> No
         return
 
     def get_bot_by_delivery_email(email: str) -> Any:
-        return UserProfile.objects.select_related().get(
-            delivery_email__iexact=email.strip(), realm=internal_realm
-        )
+        return UserProfile.objects.get(delivery_email__iexact=email.strip(), realm=internal_realm)
 
     notification_bot = get_bot_by_delivery_email(settings.NOTIFICATION_BOT)