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/test_request.py

@@ -0,0 +1,17 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+from vllm.v1.request import RequestStatus
+
+
+def test_request_status_fmt_str():
+    """Test that the string representation of RequestStatus is correct."""
+    assert f"{RequestStatus.WAITING}" == "WAITING"
+    assert f"{RequestStatus.WAITING_FOR_FSM}" == "WAITING_FOR_FSM"
+    assert f"{RequestStatus.WAITING_FOR_REMOTE_KVS}" == "WAITING_FOR_REMOTE_KVS"
+    assert f"{RequestStatus.WAITING_FOR_STREAMING_REQ}" == "WAITING_FOR_STREAMING_REQ"
+    assert f"{RequestStatus.RUNNING}" == "RUNNING"
+    assert f"{RequestStatus.PREEMPTED}" == "PREEMPTED"
+    assert f"{RequestStatus.FINISHED_STOPPED}" == "FINISHED_STOPPED"
+    assert f"{RequestStatus.FINISHED_LENGTH_CAPPED}" == "FINISHED_LENGTH_CAPPED"
+    assert f"{RequestStatus.FINISHED_ABORTED}" == "FINISHED_ABORTED"
+    assert f"{RequestStatus.FINISHED_IGNORED}" == "FINISHED_IGNORED"