Harden PR preview workflow against transient deploy/comment failures

[Copilot]

PR previews occasionally failed after a successful deploy because the visual/comment step errored, causing the job to report failure despite publishing the preview.

• Deploy step resilience: Run the rossjrw/pr-preview-action deploy step with continue-on-error so transient post-deploy/comment issues don’t abort the job.
• Explicit validation: Add a validation step that requires a deployment URL and treats non-success outcomes as comment/visual transient errors when the URL exists.
• Success comment gating: Post the success comment only after validation confirms a usable preview URL.

Example (workflow excerpt):

- name: Deploy preview
  id: deploy-preview
  continue-on-error: true
  uses: rossjrw/pr-preview-action@v1.4.8
  with:
    source-dir: ./preview-staging
    preview-branch: gh-pages
    umbrella-dir: pr-preview
    action: deploy

- name: Validate preview deployment
  id: validate-deploy
  run: |
    DEPLOY_OUTCOME="${{ steps.deploy-preview.outcome }}"
    DEPLOY_URL="${{ steps.deploy-preview.outputs.deployment-url }}"
    if [ -z "$DEPLOY_URL" ]; then exit 1; fi
    if [ "$DEPLOY_OUTCOME" != "success" ]; then
      echo "Preview URL exists; treating as transient post-deploy error."
    fi
    echo "status=ok" >> "$GITHUB_OUTPUT"

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