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/utils_/test_async_utils.py

@@ -0,0 +1,42 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import asyncio
+from collections.abc import AsyncIterator
+
+import pytest
+
+from vllm.utils.async_utils import merge_async_iterators
+
+
+async def _mock_async_iterator(idx: int):
+    try:
+        while True:
+            yield f"item from iterator {idx}"
+            await asyncio.sleep(0.1)
+    except asyncio.CancelledError:
+        print(f"iterator {idx} cancelled")
+
+
+@pytest.mark.asyncio
+async def test_merge_async_iterators():
+    iterators = [_mock_async_iterator(i) for i in range(3)]
+    merged_iterator = merge_async_iterators(*iterators)
+
+    async def stream_output(generator: AsyncIterator[tuple[int, str]]):
+        async for idx, output in generator:
+            print(f"idx: {idx}, output: {output}")
+
+    task = asyncio.create_task(stream_output(merged_iterator))
+    await asyncio.sleep(0.5)
+    task.cancel()
+    with pytest.raises(asyncio.CancelledError):
+        await task
+
+    for iterator in iterators:
+        try:
+            await asyncio.wait_for(anext(iterator), 1)
+        except StopAsyncIteration:
+            # All iterators should be cancelled and print this message.
+            print("Iterator was cancelled normally")
+        except (Exception, asyncio.CancelledError) as e:
+            raise AssertionError() from e