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/v1/shutdown/conftest.py

@@ -0,0 +1,26 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import os
+from collections.abc import Iterable
+from pathlib import Path
+
+import pytest
+
+from vllm.platforms import current_platform
+
+
+@pytest.fixture
+def rocm_sitecustomize_factory(monkeypatch, tmp_path: Path):
+    """Return a function that installs a given sitecustomize payload."""
+    if not current_platform.is_rocm():
+        return lambda _: None
+
+    def install(lines: Iterable[str]) -> None:
+        sc = tmp_path / "sitecustomize.py"
+        sc.write_text("\n".join(lines) + "\n")
+        monkeypatch.setenv(
+            "PYTHONPATH",
+            os.pathsep.join(filter(None, [str(tmp_path), os.getenv("PYTHONPATH")])),
+        )
+
+    return install