Airflow: Complete the DAG code example in Airflow stock market tutorial

[coderabbitai]

Description

The final code example in docs/integrate/airflow/import-stock-market-data.md contains incomplete task implementations with empty function bodies. Readers copying this code will encounter errors.

Current Issue

The code block at the end of the tutorial shows:

• Empty @task function bodies for download_yfinance_data and prepare_data
• Fragmented presentation of the complete DAG structure

Requested Improvement

Provide a complete, runnable DAG example that includes:

• Full task implementations with proper function bodies
• Complete download_yfinance_data function that fetches data via yfinance
• Complete prepare_data function that transforms data into the expected format
• Proper error handling and logging
• All components in a single, copy-pasteable code block

Context

• File: docs/integrate/airflow/import-stock-market-data.md
• PR: Airflow: Add tutorials from community forum #279
• Comment: Airflow: Add tutorials from community forum #279 (comment)
• Requested by: @amotl