feat: Naive support Spec V2 + Constrained Decoding (#13425)

Signed-off-by: Ubospica <ubospica@gmail.com>
Co-authored-by: Liangsheng Yin <lsyincs@gmail.com>

python/sglang/srt/managers/scheduler_output_processor_mixin.py

@@ -395,10 +395,16 @@ class SchedulerOutputProcessorMixin:
                     logits_output.hidden_states[i].cpu().clone().tolist()
                 )
 
-            if req.grammar is not None and batch.spec_algorithm.is_none():
+            if req.grammar is not None:
                 # FIXME: this try-except block is for handling unexpected xgrammar issue.
                 try:
-                    req.grammar.accept_token(next_token_id)
+                    if batch.spec_algorithm.is_none():
+                        # Normal decode: single token
+                        req.grammar.accept_token(next_token_id)
+                    elif batch.is_v2_eagle:
+                        # Speculative decode: next_token_id is a list of accepted tokens
+                        for token_id in next_token_id:
+                            req.grammar.accept_token(token_id)
                 except ValueError as e:
                     # Grammar accept_token can raise ValueError if the token is not in the grammar.
                     # This can happen if the grammar is not set correctly or the token is invalid.