chore: operational fix for CI timeout

[sonupreetam]

Summary

Verification was succeeding, but the step never "finished" because GitHub Actions was struggling to flush 1.7MB of JSON into the log.

• The "Verify SBOM" step in the sign-and-verify workflow was hanging/timing out because
  cosign verify-attestation dumps the entire verified attestation payload (~1.7MB for the
  SBOM DSSE envelope) to stdout, which GitHub Actions struggles to buffer and render in the
  step log.
• Redirect stdout to /dev/null on all three verify-attestation steps (SLSA provenance,
  SBOM, vulnerability scan).
• Verification still works via exit code — cosign exits non-zero
  on failure. The --verbose debug trace (written to stderr) remains visible in the logs.
• This step had finished during previous testing.

Updates

• reusable_sign_and_verify.yml

Review Hints

• Run the sign-and-verify workflow and confirm all three verify steps complete without hanging
• Check the test run: https://github.com/sonupreetam/image-publish-test/actions/runs/22500307259