feat(datasets): add tasks subcommand with file task support and improved output

[berstpander]

Summary

• Add rock datasets tasks CLI command to list task IDs under a dataset split
• Fix task listing to recognize both directory and file tasks
• Improve output formatting with visual separation from logs

closes #874

Changes

CLI

• Add rock datasets tasks with --org, --dataset, --split, --offset, --limit args
• Add separator lines and #Task name header for readable output

SDK

• Add list_dataset_tasks to DatasetClient and registry layers
• Add _extract_tasks_from_split method to merge directory + file tasks
• Strip file suffix (e.g., task-001.json → task-001)
• Ignore placeholder objects and nested paths

Tests

• Unit tests for CLI command, client, and OSS registry
• Tests for directory + file task merging scenarios

Test Plan

• uv run pytest tests/unit/datasets/ -v passes (15 tests)

Usage Example

rock datasets tasks \                                                                                                                             
  --bucket my-bucket \
  --org my-org --dataset my-dataset --split train                                                                                                 
                                                                                                                                                  
Output:                                                                                                                                           
                                                                                                                                                  
================================================================================                                                                  
Dataset: my-org/my-dataset  Split: train  Total: 5  Shown: 5                                                                                      
================================================================================                                                                  
#Task name                                                                                                                                        
----------                                                                                                                                        
environment                                                                                                                                       
instruction                                                                                                                                       
solution                                                                                                                                          
task                                                                                                                                              
tests