Add vLLM v0.18.1 source tree with KV transfer abort fix

third_party/vllm/ now tracked in git for direct patch management.
Based on vLLM v0.18.1 release with one patch applied:

  vllm/v1/core/sched/scheduler.py:
    Replace fatal assert with graceful skip when KV transfer callback
    arrives for an already-aborted request during PD disaggregated serving.

Future vLLM modifications should be made directly in third_party/vllm/
and committed normally. The patches/ directory is kept as documentation
of what changed from upstream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/vllm/tests/utils_/test_jsontree.py

@@ -0,0 +1,32 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+from vllm.utils.jsontree import json_count_leaves
+
+
+def test_json_count_leaves():
+    """Test json_count_leaves function from jsontree utility."""
+
+    # Single leaf values
+    assert json_count_leaves(42) == 1
+    assert json_count_leaves("hello") == 1
+    assert json_count_leaves(None) == 1
+
+    # Empty containers
+    assert json_count_leaves([]) == 0
+    assert json_count_leaves({}) == 0
+    assert json_count_leaves(()) == 0
+
+    # Flat structures
+    assert json_count_leaves([1, 2, 3]) == 3
+    assert json_count_leaves({"a": 1, "b": 2}) == 2
+    assert json_count_leaves((1, 2, 3)) == 3
+
+    # Nested structures
+    nested_dict = {"a": 1, "b": {"c": 2, "d": 3}}
+    assert json_count_leaves(nested_dict) == 3
+
+    nested_list = [1, [2, 3], 4]
+    assert json_count_leaves(nested_list) == 4
+
+    mixed_nested = {"list": [1, 2], "dict": {"x": 3}, "value": 4}
+    assert json_count_leaves(mixed_nested) == 4