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/requirements/docs.txt

@@ -0,0 +1,17 @@
+mkdocs<2.0.0
+mkdocs-api-autonav
+mkdocs-material
+mkdocstrings-python
+mkdocs-gen-files
+mkdocs-awesome-nav
+mkdocs-glightbox
+mkdocs-git-revision-date-localized-plugin
+mkdocs-minify-plugin
+mkdocs-redirects
+regex
+ruff
+pydantic
+
+# For generating argparse docs.
+# Adding requirements here should only be used as a last resort.
+msgspec  # Need for multiple inheritance involving msgspec.Struct