Add GitHub Actions step to validate Postmark credentials

[Copilot]

Tests require POSTMARK_API_TOKEN and SENDER_EMAIL secrets to send test emails, but fail cryptically when these aren't configured.

Changes

• Added pre-test validation step in .github/workflows/tests.yml that checks both secrets are set
• Fails fast with clear error message before test execution if credentials missing
• Uses environment variables to prevent secret exposure in logs

- name: Check Postmark credentials
  env:
    POSTMARK_API_TOKEN: ${{ secrets.POSTMARK_API_TOKEN }}
    SENDER_EMAIL: ${{ secrets.SENDER_EMAIL }}
  run: |
    if [ -z "$POSTMARK_API_TOKEN" ]; then
      echo "Error: POSTMARK_API_TOKEN is not set in repository secrets"
      exit 1
    fi
    if [ -z "$SENDER_EMAIL" ]; then
      echo "Error: SENDER_EMAIL is not set in repository secrets"
      exit 1
    fi

Original prompt

Create a separate action to check if the postmark informationa are set a

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