fix: Prevent scientific notation in row validation for Db2 REAL/DOUBLE columns

[nj1973]

Description of changes

This PR prevents scientific notation in row validation for Db2 REAL/DOUBLE columns. To do this I changed cast(... to varchar) to to_char(...). In my tests, with this change in place, I did not see scientific notation used.

I cannot add col_float32/64 to tests though due to general differences in string conversion output. I added an internal doc capturing that for future reference.

Issues to be closed

Closes #1722

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.

[nj1973]

/gcbrun