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/models/multimodal/pooling/conftest.py

@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Pytest configuration for vLLM pooling tests."""
+
+import pytest
+
+from vllm.platforms import current_platform
+
+
+@pytest.fixture
+def siglip_attention_config():
+    """Return attention config for SigLIP tests on ROCm.
+
+    On ROCm, SigLIP tests require FLEX_ATTENTION backend.
+    """
+    if current_platform.is_rocm():
+        return {"backend": "FLEX_ATTENTION"}
+    return None