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/.buildkite/test_areas/weight_loading.yaml

@@ -0,0 +1,25 @@
+group: Weight Loading
+depends_on: 
+  - image-build
+steps:
+- label: Weight Loading Multiple GPU  # 33min
+  timeout_in_minutes: 45
+  working_dir: "/vllm-workspace/tests"
+  num_devices: 2
+  optional: true
+  source_file_dependencies:
+  - vllm/
+  - tests/weight_loading
+  commands:
+    - bash weight_loading/run_model_weight_loading_test.sh -c weight_loading/models.txt
+
+# - label: Weight Loading Multiple GPU - Large Models # optional
+#   working_dir: "/vllm-workspace/tests"
+#   num_devices: 2
+#   device: a100
+#   optional: true
+#   source_file_dependencies:
+#   - vllm/
+#   - tests/weight_loading
+#   commands:
+#     - bash weight_loading/run_model_weight_loading_test.sh -c weight_loading/models-large.txt