Allow shipping tax rate to be determined by a callable

[ryanmitchell]

Adds support for a callable config value for shipping_rate_tax_calculation, allowing applications to provide custom logic to determine which tax class should be applied to shipping rates.

Previously the config only accepted 'default' (system default tax class) or 'highest' (highest tax rate among cart lines). Any value other than 'default' would fall through to the highest-rate resolver, with no way to supply arbitrary logic.

Changes

• ShippingRate::getShippingOption() — when shipping_rate_tax_calculation is set to a callable (closure or invokable class), it is called with the Cart instance and the return value is used as the resolved TaxClass

Usage

// config/shipping-tables.php
'shipping_rate_tax_calculation' => function (\Lunar\Models\Cart $cart): \Lunar\Models\TaxClass {
    // custom logic — e.g. return a fixed tax class for B2B customers
    return $cart->user?->isBusinessCustomer()
        ? TaxClass::where('name', 'B2B')->first()
        : TaxClass::getDefault();
},

An invokable class is also supported:

'shipping_rate_tax_calculation' => [\App\Shipping\CustomTaxClassResolver, 'handle'],

[alecritson]

Thanks @ryanmitchell I think we should avoid closures in config file as this will stop serialization, also maybe instead (or on top) of having to provide a handle [MyTaxRateCalculator::class, 'calculate'] we make use of invokable classes?

Finally I think if we can simply or tidy up the nested if statements that would be ideal.

[ryanmitchell]

@alecritson nested ifs are removed.

It already supports invokeable classes (eg 'shipping_rate_tax_calculation' => MyInvokableClass::class).