Fix dry_run for veda-discover workflow

[botanical]

Dry run configuration is not working for our discovery workflow.

Acceptance Criteria

• "dry_run": true in veda-discover run config completes without ingesting STAC items to the ingestor API
• Update the logs to clearly indicate dry-run mode and show representative items that would be processed
• dry_run": false (or omitted) preserves current behavior
• dry_run is documented in veda_discover DAG docs
• Tests done to test functionality