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/docs/serving/integrations/langchain.md

@@ -0,0 +1,32 @@
+# LangChain
+
+vLLM is also available via [LangChain](https://github.com/langchain-ai/langchain) .
+
+To install LangChain, run
+
+```bash
+pip install langchain langchain_community -q
+```
+
+To run inference on a single or multiple GPUs, use `VLLM` class from `langchain`.
+
+??? code
+
+    ```python
+    from langchain_community.llms import VLLM
+
+    llm = VLLM(
+        model="Qwen/Qwen3-4B",
+        trust_remote_code=True,  # mandatory for hf models
+        max_new_tokens=128,
+        top_k=10,
+        top_p=0.95,
+        temperature=0.8,
+        # for distributed inference
+        # tensor_parallel_size=...,
+    )
+
+    print(llm("What is the capital of France ?"))
+    ```
+
+Please refer to this [Tutorial](https://python.langchain.com/docs/integrations/llms/vllm) for more details.