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/.github/scripts/cleanup_pr_body.sh

@@ -0,0 +1,50 @@
+#!/bin/bash
+
+set -eu
+
+# ensure 1 argument is passed
+if [ "$#" -ne 1 ]; then
+    echo "Usage: $0 <pr_number>"
+    exit 1
+fi
+
+PR_NUMBER=$1
+OLD=/tmp/orig_pr_body.txt
+NEW=/tmp/new_pr_body.txt
+
+gh pr view --json body --template "{{.body}}" "${PR_NUMBER}" > "${OLD}"
+cp "${OLD}" "${NEW}"
+
+# Remove markdown comments (like the <!-- markdownlint-disable --> at the start)
+sed -i '/<!--.*-->$/d' "${NEW}"
+
+# Remove "PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED."
+sed -i '/PLEASE FILL IN THE PR DESCRIPTION HERE.*$/d' "${NEW}"
+
+# Remove all lines after and including "**BEFORE SUBMITTING, PLEASE READ THE CHECKLIST BELOW AND FILL IN THE DESCRIPTION ABOVE**"
+sed -i '/\*\*BEFORE SUBMITTING, PLEASE READ.*\*\*/,$d' "${NEW}"
+
+# Remove HTML <details> section that includes <summary> text of "PR Checklist (Click to Expand)"
+python3 - <<EOF
+import regex as re
+
+with open("${NEW}", "r") as file:
+    content = file.read()
+
+pattern = re.compile(r'(---\n\n)?<details>.*?<summary>.*?PR Checklist \(Click to Expand\).*?</summary>.*?</details>', re.DOTALL)
+content = re.sub(pattern, '', content)
+
+with open("${NEW}", "w") as file:
+    file.write(content)
+EOF
+
+# Run this only if ${NEW} is different than ${OLD}
+if ! cmp -s "${OLD}" "${NEW}"; then
+    gh pr edit --body-file "${NEW}" "${PR_NUMBER}"
+    echo
+    echo "Updated PR body:"
+    echo
+    cat "${NEW}"
+else
+    echo "No changes needed"
+fi