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/kernels/helion/test_helion_available.py

@@ -0,0 +1,45 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Tests for Helion kernel availability and basic functionality.
+
+This module demonstrates the pattern for testing optional Helion kernels.
+Tests in this directory will be skipped if Helion is not installed.
+"""
+
+import pytest
+
+from vllm.utils.import_utils import has_helion
+
+# Skip entire module if helion is not available
+if not has_helion():
+    pytest.skip(
+        "Helion is not installed. Install with: pip install vllm[helion]",
+        allow_module_level=True,
+    )
+
+import helion
+import helion.language as hl
+import torch
+
+
+def test_helion_kernel_compilation_smoke():
+    """Smoke test: compile and run a simple Helion kernel."""
+
+    @helion.kernel(autotune_effort="none")
+    def add_kernel(x: torch.Tensor, y: torch.Tensor) -> torch.Tensor:
+        out = torch.empty_like(x)
+        for tile in hl.tile(x.size()):
+            out[tile] = x[tile] + y[tile]
+        return out
+
+    # Create test tensors
+    x = torch.randn(1024, device="cuda", dtype=torch.float32)
+    y = torch.randn(1024, device="cuda", dtype=torch.float32)
+
+    # Run the helion kernel
+    result = add_kernel(x, y)
+
+    # Verify correctness
+    expected = x + y
+    assert torch.allclose(result, expected), "Helion kernel output mismatch"