Add LEVELS agent benchmark notebook for DSI Explorer

[Ha-Tien-121]

Benchmarking DSI explorer on tasks that covers:

Three complexity levels:

• Level 1: single API/tool calls
• Level 2: multi-step workflows
• Level 3: hypothesis-level discovery tasks

Cover five data-management categories:

• Search
• Data Discovery
• Data Preparation
• Metadata Generation
• Analysis & Summarization.