fix(ci): fix mirror SHA detection in e2e-label-help workflow

The branches API endpoint cannot handle ref names with slashes (e.g.
pull-request/2223). When the API returns a 404, gh api writes the error
JSON to stdout before exiting non-zero, so the || fallback never fires.
This causes mirror_sha to contain raw JSON like {"message":"Branch not
found",...} and the comment displays `{"messa` as the SHA.

Switch to the git/ref/heads/ endpoint which handles slashes natively,
and add a regex guard to reset mirror_sha to empty when it does not
look like a valid 40-char hex SHA.

Assisted-By: 🤖 Claude Code
Signed-off-by: Roland Huß <roland@ro14nd.de>
Signed-off-by: Roland Huß <rhuss@redhat.com>

Author: rhuss

.github/workflows/e2e-label-help.yml

@@ -59,7 +59,8 @@ jobs:
           esac
 
           mirror_ref="pull-request/$PR_NUMBER"
-          mirror_sha=$(gh api "repos/$GH_REPO/branches/$mirror_ref" --jq '.commit.sha' 2>/dev/null || echo "")
+          mirror_sha=$(gh api "repos/$GH_REPO/git/ref/heads/$mirror_ref" --jq '.object.sha' 2>/dev/null || true)
+          if [[ ! "$mirror_sha" =~ ^[0-9a-f]{40}$ ]]; then mirror_sha=""; fi
           short_pr=${PR_HEAD_SHA:0:7}
 
           if [ -z "$mirror_sha" ]; then