fix: custom query row validation on Db2 loses precision and scale

[anzzyspeaksgit]

Summary

Fixes an issue where custom query row validation on Db2 loses decimal precision and scale because it was ignoring cursor.description values.

Problem

Closes #1706

Solution

Updated _get_type() to receive the entire column tuple from cursor.description rather than just the type string, and extracted precision and scale (at indexes 4 and 5 respectively) when returning an Ibis Decimal data type. This mimics how PostgreSQL and Oracle extract type sizes.

Testing

• Code updated
• Lint passed

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