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/standalone_tests/python_only_compile.sh

@@ -0,0 +1,79 @@
+#!/bin/bash
+# This script tests if the python only compilation works correctly
+# for users who do not have any compilers installed on their system
+
+set -e
+
+merge_base_commit=$(git merge-base HEAD origin/main)
+echo "INFO: current merge base commit with main: $merge_base_commit"
+git show --oneline -s "$merge_base_commit"
+
+# test whether the metadata.json url is valid, retry each 3 minutes up to 5 times
+# this avoids cumbersome error messages & manual retries in case the precompiled wheel
+# for the given commit is still being built in the release pipeline
+meta_json_url="https://wheels.vllm.ai/$merge_base_commit/vllm/metadata.json"
+echo "INFO: will use metadata.json from $meta_json_url"
+
+for i in {1..5}; do
+    echo "Checking metadata.json URL (attempt $i)..."
+    if curl --fail "$meta_json_url" > metadata.json; then
+        echo "INFO: metadata.json URL is valid."
+        # check whether it is valid json by python (printed to stdout)
+        if python3 -m json.tool metadata.json; then
+            echo "INFO: metadata.json is valid JSON. Proceeding with the check."
+            # check whether there is an object in the json matching:
+            # "package_name": "vllm", and "platform_tag" matches the current architecture
+            # see `determine_wheel_url` in setup.py for more details
+            if python3 -c "import platform as p,json as j,sys as s; d = j.load(open('metadata.json')); \
+             s.exit(int(not any(o.get('package_name') == 'vllm' and p.machine() in o.get('platform_tag') \
+             for o in d)))" 2>/dev/null; then
+                echo "INFO: metadata.json contains a pre-compiled wheel for the current architecture."
+                break
+            else
+                echo "WARN: metadata.json does not have a pre-compiled wheel for the current architecture."
+            fi
+        else
+            echo "CRITICAL: metadata.json exists but is not valid JSON, please do report in #sig-ci channel!"
+            echo "INFO: metadata.json content:"
+            cat metadata.json
+            exit 1
+        fi
+    fi
+    # failure handling & retry logic
+    if [ "$i" -eq 5 ]; then
+        echo "ERROR: metadata is still not available after 5 attempts."
+        echo "ERROR: Please check whether the precompiled wheel for commit $merge_base_commit is available."
+        echo " NOTE: If $merge_base_commit is a new commit on main, maybe try again after its release pipeline finishes."
+        echo " NOTE: If it fails, please report in #sig-ci channel."
+        exit 1
+    else
+        echo "WARNING: metadata is not available. Retrying after 5 minutes..."
+        sleep 300
+    fi
+done
+
+set -x
+
+cd /vllm-workspace/
+
+# uninstall vllm
+pip3 uninstall -y vllm
+# restore the original files
+mv src/vllm ./vllm
+
+# remove all compilers
+apt remove --purge build-essential -y
+apt autoremove -y
+
+echo 'import os; os.system("touch /tmp/changed.file")' >> vllm/__init__.py
+
+VLLM_PRECOMPILED_WHEEL_COMMIT=$merge_base_commit VLLM_USE_PRECOMPILED=1 pip3 install -vvv -e .
+
+# Run the script
+python3 -c 'import vllm'
+
+# Check if the clangd log file was created
+if [ ! -f /tmp/changed.file ]; then
+    echo "ERROR: changed.file was not created, python only compilation failed"
+    exit 1
+fi