Fix error handling around when completing an AS transaction (#8693)

changelog.d/8693.misc

+Add more type hints to the application services code.

synapse/storage/databases/main/appservice.py

@@ -261,7 +261,7 @@ class ApplicationServiceTransactionWorkerStore(
             # has probably missed some events), so whine loudly but still continue,
             # since it shouldn't fail completion of the transaction.
             last_txn_id = self._get_last_txn(txn, service.id)
-            if (txn_id + 1) != txn_id:
+            if (last_txn_id + 1) != txn_id:
                 logger.error(
                     "appservice: Completing a transaction which has an ID > 1 from "
                     "the last ID sent to this AS. We've either dropped events or "