Fix tiny misalign with previous truncation setting in tokenizer_manager (#9430)

python/sglang/srt/managers/tokenizer_manager.py

@@ -566,7 +566,7 @@ class TokenizerManager:
     ) -> None:
         """Validates that the input token count and the requested token count doesn't exceed the model's context length."""
         # FIXME: unify the length validation logic with the one in the scheduler.
-        _max_req_len = self.context_len - 1
+        _max_req_len = self.context_len
 
         input_token_num = len(input_ids) if input_ids is not None else 0
         if input_token_num >= self.context_len: