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/test_outputs.py

@@ -0,0 +1,21 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+
+from vllm.outputs import RequestOutput
+
+pytestmark = pytest.mark.cpu_test
+
+
+def test_request_output_forward_compatible():
+    output = RequestOutput(
+        request_id="test_request_id",
+        prompt="test prompt",
+        prompt_token_ids=[1, 2, 3],
+        prompt_logprobs=None,
+        outputs=[],
+        finished=False,
+        example_arg_added_in_new_version="some_value",
+    )
+    assert output is not None