Let unittest._ShouldStop propagate through retry() so subTest+failfast works (#26616)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

python/sglang/srt/utils/common.py

@@ -72,6 +72,7 @@ from typing import (
     Union,
 )
 from unittest import SkipTest
+from unittest.case import _ShouldStop
 from urllib.parse import unquote, urlparse
 
 import numpy as np
@@ -2771,6 +2772,13 @@ def retry(
         except SkipTest:
             # Do NOT retry skipped tests - used in CI and unittest
             raise
+        except _ShouldStop:
+            # `unittest.case._ShouldStop` is raised by `subTest.__exit__`
+            # when a subtest fails/skips and `result.failfast` is True
+            # (CI invokes `python3 file.py -f`). It signals the outer
+            # `testPartExecutor` to stop the test method cleanly; do
+            # NOT retry, just propagate so unittest handles it.
+            raise
         except Exception as e:
             traceback.print_exc()