test(internal): extend handler-key assertion with 8 backup handlers

After the master merge, build_settings_handlers returns 16 keys (the
8 from #1381 plus the 8 auto-backup handlers added by this PR).
test_returns_all_handler_keys was asserting only master's 8 — update
to the union so the test passes again.

Author: kingpanther13

tests/src/unit/test_stdio_settings_sidecar.py

@@ -81,6 +81,15 @@ def test_returns_all_handler_keys(self) -> None:
             "settings_info",
             "get_feature_flags",
             "save_feature_flags",
+            # Auto-backup handlers (#1288).
+            "list_backups",
+            "view_backup",
+            "diff_backup",
+            "restore_backup",
+            "delete_backup",
+            "delete_backups_bulk",
+            "get_backup_config",
+            "save_backup_config",
         }
 
     def test_get_tools_reads_cache_when_server_is_none(