Fix: Support for DESCRIBE and SHOW COLUMNS queries

[paolobiancalana]

I made two important improvements:

1. Schema query support:

• Added specific handling for DESCRIBE, DESC, SHOW COLUMNS and SHOW FIELDS
• Implemented correct formatting of these query results
• Added conversion of NULL values to readable "NULL" strings

2. Code architecture improvements:

• Rearranged input validation to improve tests
• Added query normalization for more reliable detection
• Improved handling of empty results

Tests
Passed all automated tests and manually tested with:

• DESCRIBE bookings
• SHOW COLUMNS FROM bookings
• DESC bookings
• SHOW FIELDS FROM bookings

Impact
This change significantly improves the user experience with LLM models that tend to use these commands to explore database structure, eliminating frustration and confusion caused by "Unread result found" errors.