[HiCache] Read the in-flight buffer backup's node id from its snapshot in sanity_check (#40013)

Co-authored-by: Pranjal Shankhdhar <pranjalssh@meta.com>
Co-authored-by: Jialin Ouyang <Jialin.Ouyang@gmail.com>

python/sglang/srt/mem_cache/unified_radix_cache.py

@@ -3548,7 +3548,7 @@ class UnifiedRadixCache(BasePrefixCache):
         # can resolve + validate them without reaching into Controller state.
         if self.buffer_pipeline is not None:
             ongoing_write_through = [
-                (nid, entry.intent.node_id)
+                (nid, entry.intent.snapshot.node_id)
                 for nid, entry in self.buffer_pipeline.ongoing_write_through.items()
             ]
         else: