fix: print error message before silent exit in edit-issue-labels.sh

[Dreamstick9]

Fixes: #69913

What this does

Fixes a silent exit 1 in scripts/edit-issue-labels.sh when the script is called with no --add-label or --remove-label arguments.

Why now

The Claude Issue Triage workflow calls edit-issue-labels.sh via the triage-issue command. If Claude ever invokes the script with no label arguments — which the triage prompt explicitly allows on comment events ("it's okay to make no changes if nothing applies") — the script exits with code 1 and prints nothing to stdout or stderr.

Because the workflow runs with set -euo pipefail, this causes the step to fail silently. The GitHub Actions log shows a non-zero exit with zero diagnostic output, making the failure impossible to diagnose without reading the source.

Every other error path in the same script prints a message to stderr before exiting:

# Line 15 — prints before exit  [header-1](#header-1)
echo "Error: no issue number in event payload" >&2; exit 1  
  
# Line 34 — prints before exit  [header-2](#header-2)
echo "Error: unknown argument (only --add-label and --remove-label are accepted)" >&2; exit 1  
  
# Line 40 — silent, prints nothing ← this PR  [header-3](#header-3)
if [[ ${#ADD_LABELS[@]} -eq 0 && ${#REMOVE_LABELS[@]} -eq 0 ]]; then  
  exit 1  
fi

What changed

Added a single echo to stderr before the exit 1 at line 40–42 of scripts/edit-issue-labels.sh, making it consistent with every other error path in the file:

 if [[ ${#ADD_LABELS[@]} -eq 0 && ${#REMOVE_LABELS[@]} -eq 0 ]]; then  
+  echo "Error: at least one --add-label or --remove-label argument is required" >&2  
   exit 1  
 fi

How we know it works

Reproduces the silent failure before the fix:

export GITHUB_EVENT_PATH=$(mktemp)  
echo '{"issue":{"number":1}}' > "$GITHUB_EVENT_PATH"  
bash scripts/edit-issue-labels.sh   # exits 1, no output

After the fix, the same invocation prints Error: at least one --add-label or --remove-label argument is required to stderr before exiting 1.

All other code paths are unchanged and are passing valid labels still works normally. ShellCheck passes on the modified file.

After merge

No workflow triggers or backfill needed. The fix is purely defensive — it makes a latent silent failure loud so it can be diagnosed if it ever occurs.