Function validate_arrow_schema_against_mappings 

pub fn validate_arrow_schema_against_mappings(
    schema: &Schema,
    mappings: &[SchemaMapping],
) -> Result<()>

Validates a runtime Arrow schema against planned Arrow-side schema mappings.

This is a strict schema-contract check for callers that plan once and later want to confirm a runtime schema still matches that plan before writing. It verifies field count, field order, planned Arrow index, field name, Arrow data type compatibility, and Arrow nullability.

This function does not inspect row values. A nullable runtime value in a non-nullable SQL Server target column is still a value-conversion error and is reported by writer paths that inspect rows.