fix(unified radix cache w/ hicache): backup ancestor nodes before leaf in write_back eviction (#24585)

Co-authored-by: Zhangheng <hzh0425@apache.org>

python/sglang/srt/mem_cache/unified_radix_cache.py

@@ -390,6 +390,12 @@ class UnifiedRadixCache(BasePrefixCache):
         for component in self._components_tuple:
             component.drive_eviction(params=params, tracker=tracker)
 
+        if (
+            self.cache_controller is not None
+            and self.cache_controller.write_policy == "write_back"
+        ):
+            self.writing_check(write_back=True)
+
         self.update_eviction_metrics(sum(tracker.values()), start_time)
         return EvictResult(
             num_tokens_evicted=tracker[BASE_COMPONENT_TYPE],
@@ -1653,6 +1659,11 @@ class UnifiedRadixCache(BasePrefixCache):
         if self.session.any_holding_kv():
             return
 
+        write_back = (
+            self.cache_controller is not None
+            and self.cache_controller.write_policy == "write_back"
+        )
+
         errors: list[str] = []
         E = errors.append
         all_nodes = self._collect_all_nodes()
@@ -1709,7 +1720,7 @@ class UnifiedRadixCache(BasePrefixCache):
                 p_hst = node.parent.component_data[FCT].host_value is not None
                 if full_dev and not p_dev:
                     E(f"node {nid} device present but parent {node.parent.id} evicted")
-                if full_hst and not p_hst:
+                if full_hst and not p_hst and not write_back:
                     E(f"node {nid} backed up but parent {node.parent.id} not backed up")
 
             # Lock hierarchy and counters must stay sane.