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_system_utils.py

@@ -0,0 +1,19 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import tempfile
+from pathlib import Path
+
+from vllm.utils.system_utils import unique_filepath
+
+
+def test_unique_filepath():
+    temp_dir = tempfile.mkdtemp()
+    path_fn = lambda i: Path(temp_dir) / f"file_{i}.txt"
+    paths = set()
+    for i in range(10):
+        path = unique_filepath(path_fn)
+        path.write_text("test")
+        paths.add(path)
+    assert len(paths) == 10
+    assert len(list(Path(temp_dir).glob("*.txt"))) == 10