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/deployment/integrations/aibrix.md

@@ -0,0 +1,5 @@
+# AIBrix
+
+[AIBrix](https://github.com/vllm-project/aibrix) is a cloud-native control plane that integrates with vLLM to simplify Kubernetes deployment, scaling, routing, and LoRA adapter management for large language model inference.
+
+For installation and usage instructions, please refer to the [AIBrix documentation](https://aibrix.readthedocs.io/).