[8.19] Add pull request job to verify transport versions backported via main (#134568)

.buildkite/pipelines/pull-request/validate-transport-version-backport.yml

@@ -0,0 +1,5 @@
+config:
+  skip-target-branches: "main"
+steps:
+  - label: validate-transport-version-backport
+    command: .buildkite/scripts/validate-transport-version-backport.sh

.buildkite/scripts/validate-transport-version-backport.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+set -euo pipefail
+
+echo "--- Looking for transport version changes"
+
+# Get any changes in this pull request to transport definitions
+git fetch origin "${BUILDKITE_PULL_REQUEST_BASE_BRANCH}" --quiet
+changed_files=$(git diff --name-only "origin/${BUILDKITE_PULL_REQUEST_BASE_BRANCH}" | grep -E "server/src/main/resources/transport/definitions/.*\.csv" || true)
+
+if [[ -z "${changed_files}" ]]; then
+  echo "No transport version changes detected."
+  exit 0
+fi
+
+# Compare those files against the main branch to ensure they are the same
+git fetch origin main --quiet
+while IFS= read -r file; do
+  if ! git diff --quiet origin/main -- "${file}"; then
+      echo "Changes to transport definition [${file}] missing from main branch."
+      echo "Transport changes must first be merged to main before being backported."
+      exit 1
+  fi
+done <<< "${changed_files}"
+
+echo "All transport changes exist in main branch."