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

[nj1973]

Description of changes

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

Updated _get_type() to receive the entire column tuple, rather than just the type string, and extracted precision and scale. This mimics how PostgreSQL and Oracle backends extract type sizes.

Issues to be closed

Closes #1706

Checklist

• I have followed the CONTRIBUTING Guide.
• I have commented my code, particularly in hard-to-understand areas
• I have updated any relevant documentation to reflect my changes, if applicable
• I have added unit and/or integration tests relevant to my change as needed
• I have already checked locally that all unit tests and linting are passing (use the tests/local_check.sh script)
• I have manually executed end-to-end testing (E2E) with the affected databases/engines

[nj1973]

/gcbrun

[sundar-mudupalli-work]

Thank you. great work