TC Merge Gate: treat draft tc/pr PRs as non-blocking - #7924

Check TC PRs was failing even when testcase PRs were still in draft, which blocked engine PRs prematurely. This change narrows the gate so only open non-draft testcase PRs block merge.

• Scope

  • Update .github/workflows/tc-merge-gate.yml gate logic for tc/pr-<PR_NUMBER> checks in:
    • CUBRID/cubrid-testcases
    • CUBRID/cubrid-testcases-private-ex

• Behavior change

  • open + draft TC PR → pass (non-blocking, notice)
  • open + ready for review TC PR → fail (blocking, error)
  • no open TC PR / closed / merged → pass (unchanged)

• Message alignment

  • Adjust failure and PR comment text to explicitly refer to non-draft open TC PRs.

if [ "${IS_DRAFT}" == "true" ]; then
  echo "::notice::${TC_REPO}: TC PR is open (draft): ${URL} — pass"
else
  echo "::error::${TC_REPO}: TC PR is open: ${URL}"
  OVERALL_PASS=false
fi