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/entrypoints/openai/test_oot_registration.py

@@ -0,0 +1,42 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from ...utils import VLLM_PATH, RemoteOpenAIServer
+
+chatml_jinja_path = VLLM_PATH / "examples/template_chatml.jinja"
+assert chatml_jinja_path.exists()
+
+
+def run_and_test_dummy_opt_api_server(model, tp=1):
+    # the model is registered through the plugin
+    server_args = [
+        "--gpu-memory-utilization",
+        "0.10",
+        "--dtype",
+        "float32",
+        "--chat-template",
+        str(chatml_jinja_path),
+        "--load-format",
+        "dummy",
+        "-tp",
+        f"{tp}",
+    ]
+    with RemoteOpenAIServer(model, server_args) as server:
+        client = server.get_client()
+        completion = client.chat.completions.create(
+            model=model,
+            messages=[
+                {"role": "system", "content": "You are a helpful assistant."},
+                {"role": "user", "content": "Hello!"},
+            ],
+            temperature=0,
+        )
+        generated_text = completion.choices[0].message.content
+        assert generated_text is not None
+        # make sure only the first token is generated
+        rest = generated_text.replace("<s>", "")
+        assert rest == ""
+
+
+def test_oot_registration_for_api_server(dummy_opt_path: str):
+    run_and_test_dummy_opt_api_server(dummy_opt_path)