Enable set_display_name() to affect Kubernetes pod names for container tasks,
allowing human-readable pod names instead of auto-generated ones. This improves
cluster-side debugging and resource monitoring, especially for tasks within loops.

Benefits:

• Easier debugging: Pod names match display names, making it easier to identify
  tasks in kubectl, logs, and monitoring tools
• Resource monitoring: In ParallelFor loops, each iteration's pod has a clear
  name, enabling per-component resource usage tracking
• Better observability: Metrics and logs can be easily correlated with pipeline tasks

Changes:

• Use display_name (sanitized) as DAG task key for container tasks
• Support loop variables: task.set_display_name(f"task-{loop_var.field}")
• Ensure uniqueness and enforce Kubernetes naming constraints
• Fix producer task references in groups to use correct DAG keys