[PD-Disagg] Fix double free when prebuilt batch is aborted.

[hnyls2002]

Fix the issue: #17797

How to reproduce this bug (it only happens when grammar hit exception):

--- a/python/sglang/srt/disaggregation/decode_schedule_batch_mixin.py
+++ b/python/sglang/srt/disaggregation/decode_schedule_batch_mixin.py
@@ -110,6 +110,14 @@ class ScheduleBatchDisaggregationDecodeMixin:
         for req in self.reqs:
             self.output_ids.append(req.output_ids[-1])
             self.tree_cache.cache_unfinished_req(req)
+            from mock import Mock
+
+            # mock a exception grammar
+            req.grammar = Mock()
+            req.grammar.current_token = None
+            req.grammar.accept_token = Mock()
+            req.grammar.accept_token.side_effect = ValueError("Test Grammar Failures")
+            req.grammar.finished = Mock()
             if req.grammar is not None:
                 # FIXME: this try-except block is for handling unexpected xgrammar issue.
                 try:

Do this custom patch to the code base and launch a PD-disaggregation server, then it will enter the double free code path (but now it will raise another error of by mamba check)

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[hnyls2002]

/tag-and-rerun-ci