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/prompts/example.txt

@@ -0,0 +1,8 @@
+vLLM is a high-throughput and memory-efficient inference and serving engine for LLMs.
+Briefly describe the major milestones in the development of artificial intelligence from 1950 to 2020.
+Compare and contrast artificial intelligence with human intelligence in terms of processing information.
+Describe the basic components of a neural network and how it can be trained.
+Write a short story about a robot that dreams for the first time.
+Analyze the impact of the COVID-19 pandemic on global economic structures and future business models.
+Explain the cultural significance of the Mona Lisa painting, and how its perception might vary in Western versus Eastern societies.
+Translate the following English sentence into Japanese, French, and Swahili: 'The early bird catches the worm.'