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/detokenizer/test_disable_detokenization.py

@@ -0,0 +1,31 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+
+from vllm.entrypoints.llm import LLM
+from vllm.sampling_params import SamplingParams
+
+
+@pytest.mark.parametrize("model", ["distilbert/distilgpt2"])
+def test_computed_prefix_blocks(model: str):
+    # This test checks if the engine generates completions both with and
+    # without optional detokenization, that detokenization includes text
+    # and no-detokenization doesn't, and that both completions have the same
+    # token_ids.
+    prompt = (
+        "You are a helpful assistant. How do I build a car from cardboard and "
+        "paper clips? Is there an easy to follow video tutorial available "
+        "online for free?"
+    )
+
+    llm = LLM(model=model)
+    sampling_params = SamplingParams(max_tokens=10, temperature=0.0, detokenize=False)
+
+    outputs_no_detokenization = llm.generate(prompt, sampling_params)[0].outputs[0]
+    sampling_params.detokenize = True
+    outputs_with_detokenization = llm.generate(prompt, sampling_params)[0].outputs[0]
+
+    assert outputs_no_detokenization.text == ""
+    assert outputs_with_detokenization.text != ""
+    assert outputs_no_detokenization.token_ids == outputs_with_detokenization.token_ids