Fix build-site-metadata handling for forked PRs

[dshevtsov]

Purpose of this pull request

This pull request updates the site metadata workflow to handle forked pull requests more safely and predictably, based on behavior reproduced in AdobeDocs/commerce-php#443 and AdobeDocs/commerce-php#444.

Summary

• Check out the correct ref for forked PRs so metadata generation runs against the submitted changes
• Detect whether src/pages/adp-site-metadata.json actually changed before attempting any commit or push
• Prevent the workflow from trying to push back to forked repositories, and surface a clear error telling contributors to run npm run buildSiteMetadata locally when metadata updates are needed
• Avoid unnecessary empty commit attempts when metadata output is unchanged

Testing

• Verified the branch diff only changes .github/workflows/build-site-metadata.yml
• Confirmed the workflow now gates commit/push behavior on actual metadata changes
• Confirmed forked PRs fail with actionable guidance instead of attempting an automated push
• Validated related workflow behavior against AdobeDocs/commerce-php#443 for forked-PR testing and AdobeDocs/commerce-php#444 for same-repo PR testing

[melissag-ensemble]

looped in @timkim first since this overlaps with DEVSITE-2292 - waiting to hear back before reviewing: https://adobeio.slack.com/archives/C03AVNFAA8Y/p1775741260443359