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/.git-blame-ignore-revs

@@ -0,0 +1,4 @@
+# Migrate from `yapf` & `isort` to `ruff`
+d6953beb91da4e9c99be4c0a1304a2d24189535c
+# Convert `Optional[x]` to `x | None` and `Union[x, y]` to `x | y`
+8fcaaf6a165e661f63fc51be906bc05b0767332f