Function function_volatility 

pub fn function_volatility(
    catalog: &dyn VolatilityCatalog,
    name: &str,
    argument_count: usize,
) -> FunctionVolatility

Resolve the volatility of the implementation that can run for name.

Rust extension callbacks default to VOLATILE, while registrations with explicit options use their declared volatility. SQL routine overloads are combined conservatively: a name is only non-volatile when every overload registered under it is non-volatile. This remains correct before runtime argument coercion selects a particular overload.