fix: Silent Data Loss and Empty Row Handling in CSV Tool

[vidhan66]

Closes #3767

This PR fixes data integrity issues in the CSV tool that could cause silent data loss and inconsistent parsing.

Issues Fixed

• Empty rows (blank or whitespace-only) were sometimes treated as valid data
• Silent column drops

  csv_write discarded fields not in the provided column list
  csv_append ignored new columns not present in the existing header

Both cases could result in undetected data loss.

Changes

• Skip fully empty rows consistently
• Add strict column validation:

  Error on unexpected columns in csv_write
  Error when appending rows with new columns in csv_append