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/kernels/attention/conftest.py

@@ -0,0 +1,19 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+
+from vllm.utils.torch_utils import (
+    create_kv_caches_with_random,
+    create_kv_caches_with_random_flash,
+)
+
+
+@pytest.fixture()
+def kv_cache_factory():
+    return create_kv_caches_with_random
+
+
+@pytest.fixture()
+def kv_cache_factory_flashinfer():
+    return create_kv_caches_with_random_flash