fast regex decode

Auto-detect constant str path in regex FSM, then extend instead.

test/srt/test_robust.py

@@ -2,14 +2,13 @@ import argparse
 import random
 import string
 
+import sglang as sgl
 from sglang.test.test_utils import (
     add_common_sglang_args_and_parse,
     select_sglang_backend,
 )
 from vllm.transformers_utils.tokenizer import get_tokenizer
 
-import sglang as sgl
-
 TOKENIZER = None
 RANDOM_PREFILL_LEN = None
 RANDOM_DECODE_LEN = None