fix: avoid double KV release on disaggregated prefill grammar errors (#30937)

Co-authored-by: Shangming Cai <csmthu@gmail.com>

python/sglang/srt/disaggregation/prefill.py

@@ -693,7 +693,6 @@ class SchedulerDisaggregationPrefillMixin:
                         req.grammar.accept_token(next_token_id)
                     except ValueError as e:
                         error_message = f"Grammar accept_token failed for req {req.rid} with token {next_token_id}: {e}"
-                        release_kv_cache(req, self.tree_cache)
                         prepare_abort(
                             req,
                             error_message,
@@ -812,7 +811,8 @@ class SchedulerDisaggregationPrefillMixin:
                 undone_reqs.append(req)
             elif poll == KVPoll.Success:  # transfer done
                 release_kv_cache(req, self.tree_cache)  # unlock the tree
-                req.finished_reason = FINISH_LENGTH(length=0)
+                if not isinstance(req.finished_reason, FINISH_ABORT):
+                    req.finished_reason = FINISH_LENGTH(length=0)
                 # FIXME: clean up req's data in transfer engine
                 req.disagg_kv_sender.clear()
                 done_reqs.append(req)
@@ -884,7 +884,10 @@ class SchedulerDisaggregationPrefillMixin:
             logger.warning(error_message)
         req.time_stats.trace_ctx.abort(abort_info={"reason": error_message})
         release_kv_cache(req, self.tree_cache)  # unlock the tree
-        prepare_abort(req, error_message, status_code=HTTPStatus.INTERNAL_SERVER_ERROR)
+        if not isinstance(req.finished_reason, FINISH_ABORT):
+            prepare_abort(
+                req, error_message, status_code=HTTPStatus.INTERNAL_SERVER_ERROR
+            )
         if self.metrics_reporter.enable_metrics:
             self.metrics_collector.increment_transfer_failed_reqs()
         return exc