Function validate_fk_value 

pub fn validate_fk_value(
    engine: &MVCCEngine,
    txn_id: i64,
    fk: &ForeignKeyConstraint,
    value: &Value,
    child_table: &str,
) -> Result<()>

Pre-validate a single FK value against its parent table. Used for early validation of constant SET values in UPDATE statements to prevent dirty state in explicit transactions.

NULL values are allowed (no reference) and should be skipped by the caller.