Show actual step name in Error Logs Preview instead of generic category

[mohit-sheth]

Use the real step name (e.g. orion-udn-l3) in the preview header instead of a generic description (e.g. orion changepoint detection failure), since many different steps share the same keyword.

Summary

• For orion/changepoint failures, display a clickable link to the changepoint visualization HTML in GCS instead of the generic "Error
  Logs Preview"
• The visualization URL is constructed by probing the job's GCS artifacts directory for the matching step subfolder, handling junit
  step-name prefixes and linking directly to the HTML file when present
• Threads step_name through analyze_prow_artifacts return tuple so callers can resolve the correct artifact path
• Extracts duplicated failure_desc logic into a shared _get_failure_desc() helper in SlackMessageFetcher

[vishnuchalla]

Pre-requisite to consider before merging this: #93

[vishnuchalla]

Somehow I cannot see the file diff on this PR. But yeah, for change points we never had a Error Logs Preview. It was a bug that is fixed in the above PR.

[mohit-sheth]

closing as this was main branch 😓

[mohit-sheth]

closing in favour of #94