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/ci_envs.py

@@ -0,0 +1,52 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+These envs only work for a small part of the tests, fix what you need!
+"""
+
+import os
+from collections.abc import Callable
+from typing import TYPE_CHECKING, Any
+
+from vllm.envs import maybe_convert_bool
+
+if TYPE_CHECKING:
+    VLLM_CI_NO_SKIP: bool = False
+    VLLM_CI_DTYPE: str | None = None
+    VLLM_CI_HEAD_DTYPE: str | None = None
+    VLLM_CI_HF_DTYPE: str | None = None
+
+environment_variables: dict[str, Callable[[], Any]] = {
+    # A model family has many models with the same architecture.
+    # By default, a model family tests only one model.
+    # Through this flag, all models can be tested.
+    "VLLM_CI_NO_SKIP": lambda: bool(int(os.getenv("VLLM_CI_NO_SKIP", "0"))),
+    # Allow changing the dtype used by vllm in tests
+    "VLLM_CI_DTYPE": lambda: os.getenv("VLLM_CI_DTYPE", None),
+    # Allow changing the head dtype used by vllm in tests
+    "VLLM_CI_HEAD_DTYPE": lambda: os.getenv("VLLM_CI_HEAD_DTYPE", None),
+    # Allow changing the head dtype used by transformers in tests
+    "VLLM_CI_HF_DTYPE": lambda: os.getenv("VLLM_CI_HF_DTYPE", None),
+    # Allow control over whether tests use enforce_eager
+    "VLLM_CI_ENFORCE_EAGER": lambda: maybe_convert_bool(
+        os.getenv("VLLM_CI_ENFORCE_EAGER", None)
+    ),
+}
+
+
+def __getattr__(name: str):
+    # lazy evaluation of environment variables
+    if name in environment_variables:
+        return environment_variables[name]()
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
+
+def __dir__():
+    return list(environment_variables.keys())
+
+
+def is_set(name: str):
+    """Check if an environment variable is explicitly set."""
+    if name in environment_variables:
+        return name in os.environ
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")