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/llm/test_prompt_validation.py

@@ -0,0 +1,34 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+import torch
+
+from vllm import LLM
+
+
+def test_empty_prompt():
+    llm = LLM(model="openai-community/gpt2", enforce_eager=True)
+    with pytest.raises(ValueError, match="decoder prompt cannot be empty"):
+        llm.generate([""])
+
+
+def test_out_of_vocab_token():
+    llm = LLM(model="openai-community/gpt2", enforce_eager=True)
+    with pytest.raises(ValueError, match="out of vocabulary"):
+        llm.generate({"prompt_token_ids": [999999]})
+
+
+def test_require_mm_embeds():
+    llm = LLM(
+        model="llava-hf/llava-1.5-7b-hf",
+        enforce_eager=True,
+        enable_mm_embeds=False,
+    )
+    with pytest.raises(ValueError, match="--enable-mm-embeds"):
+        llm.generate(
+            {
+                "prompt": "<image>",
+                "multi_modal_data": {"image": torch.empty(1, 1, 1)},
+            }
+        )