fix: preserve HiCache load-back allocator headroom

test/registered/unit/mem_cache/test_radix_cache_unit.py

@@ -39,6 +39,7 @@ from sglang.srt.mem_cache.base_prefix_cache import (
     MatchPrefixParams,
 )
 from sglang.srt.mem_cache.radix_cache import RadixCache, RadixKey, TreeNode
+from sglang.test.test_utils import CustomTestCase
 
 # Test constants
 DEFAULT_PAGE_SIZE = 4
@@ -235,6 +236,16 @@ class TestTreeNode(unittest.TestCase):
         self.assertFalse(node2 < node1)
 
 
+class TestTreeNodeChildren(CustomTestCase):
+    def test_missing_child_access_does_not_create_orphan_node(self):
+        node = TreeNode()
+
+        with self.assertRaises(KeyError):
+            _ = node.children["missing"]
+
+        self.assertEqual(node.children, {})
+
+
 class TestRadixCache(unittest.TestCase):
     """Test cases for RadixCache class."""