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_hashing.py

@@ -0,0 +1,25 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import hashlib
+import pickle
+
+import pytest
+
+from vllm.utils.hashing import sha256
+
+
+@pytest.mark.parametrize("input", [(), ("abc",), (None,), (None, bool, [1, 2, 3])])
+def test_sha256(input: tuple):
+    digest = sha256(input)
+    assert digest is not None
+    assert isinstance(digest, bytes)
+    assert digest != b""
+
+    input_bytes = pickle.dumps(input, protocol=pickle.HIGHEST_PROTOCOL)
+    assert digest == hashlib.sha256(input_bytes).digest()
+
+    # hashing again, returns the same value
+    assert digest == sha256(input)
+
+    # hashing different input, returns different value
+    assert digest != sha256(input + (1,))