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_collection_utils.py

@@ -0,0 +1,47 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import pytest
+
+from vllm.utils.collection_utils import common_prefix, swap_dict_values
+
+
+@pytest.mark.parametrize(
+    ("inputs", "expected_output"),
+    [
+        ([""], ""),
+        (["a"], "a"),
+        (["a", "b"], ""),
+        (["a", "ab"], "a"),
+        (["a", "ab", "b"], ""),
+        (["abc", "a", "ab"], "a"),
+        (["aba", "abc", "ab"], "ab"),
+    ],
+)
+def test_common_prefix(inputs, expected_output):
+    assert common_prefix(inputs) == expected_output
+
+
+@pytest.mark.parametrize(
+    ("obj", "key1", "key2"),
+    [
+        # Tests for both keys exist
+        ({1: "a", 2: "b"}, 1, 2),
+        # Tests for one key does not exist
+        ({1: "a", 2: "b"}, 1, 3),
+        # Tests for both keys do not exist
+        ({1: "a", 2: "b"}, 3, 4),
+    ],
+)
+def test_swap_dict_values(obj, key1, key2):
+    original_obj = obj.copy()
+
+    swap_dict_values(obj, key1, key2)
+
+    if key1 in original_obj:
+        assert obj[key2] == original_obj[key1]
+    else:
+        assert key2 not in obj
+    if key2 in original_obj:
+        assert obj[key1] == original_obj[key2]
+    else:
+        assert key1 not in obj