[6.x] Fix infinite recursion when a non-public data method matches an augmented key

[daun]

TL;DR: A field whose name matches a protected or private method on the entry crashes a request with memory exhaustion. Augmentation only checks if the method exists, then calls it from an outside scope that can't reach it, so PHP routes it to __call() and it augments the same handle forever, resulting in an infinite loop + crash. This PR requires the method to be public for it to be considered.

Problem

When augmenting a handle, AbstractAugmented looks for a method of the same name on the entry/term/asset/etc and calls it instead of reading the stored value. I.e. a public isSold() method overrides an is_sold field.

The check here is method_exists(), which is true for both protected and private methods. However the call happens from outside the entry class: $this->data->$method(). PHP won't call the non-public method and forwards to __call() instead. Augmentables then use HasAugmentedInstance::__call(), which augments that same handle again. Nothing stops the loop, so the request runs out of memory.

Example

This adds an internal helper method for coercing a type. It never was meant for actual augmentation, but crashes.

class Product extends Entry
{
    protected function unitsPerSet(): int
    {
        return (int) $this->value('units_per_set');
    }
}

$entry->augmentedValue('units_per_set');

Fix

Only treat the method as a match if it's public/reachable. Checked via ReflectionMethod rather than is_callable() since that also returns true for unreachable methods. Cached to avoid repetitive lookups.

Scope

This only changes the lookup on the entry. The separate lookup for methods on the Augmented* class itself is left alone on purpose: those are called from inside that class, so its own protected methods are reachable and need to keep working (permalink(), mount(), updatedBy()). There's a test that covers that.

Not breaking

Before this change, a non-public method matching a key could only and in an error: either the infinite loop above or Error: Call to protected method. There was no version of this that worked so nothing can be relying on it. Public methods still override their field, exactly as before.