fix: keep queued Mooncake linker loads after abort (#38195)

Co-authored-by: huangtingwei <141888744+huangtingwei9988@users.noreply.github.com>

python/sglang/srt/mem_cache/storage/mooncake_store/mooncake_direct_linker.py

@@ -238,7 +238,8 @@ class MooncakeDirectLinker(UnifiedCacheLinker):
         return True
 
     def cancel_queued_load(self, rid: str) -> bool:
-        return self.pending_loads.pop(rid, None) is not None
+        # Already-published loads cannot be safely canceled without tree rollback.
+        return False
 
     def num_completed_loads(self) -> int:
         return self.completed_loads.qsize()