ci: fix sync job failing when release-candidate has no branch protection rules

[Copilot]

gh pr merge --auto --merge requires branch protection rules on the target branch. Since release-candidate has none, every run of the "Promote main to release candidate" workflow was failing with Pull request Protected branch rules not configured for this branch (enablePullRequestAutoMerge).

Changes

• .github/workflows/promote-main-to-release-candidate.yml: wrap the auto-merge call in a fallback — attempt --auto --merge first; if it fails (no branch protection), fall back to a direct --merge.

if ! gh pr merge "$PR_NUMBER" --auto --merge 2>/dev/null; then
  # Auto-merge requires branch protection rules; fall back to direct merge
  gh pr merge "$PR_NUMBER" --merge
fi

The conflict-detection and Copilot conflict-resolution paths are unchanged.

Target branch

• This PR creates, updates, or deletes Dataverse data and targets release-candidate.
• This PR is read-only and targets main.