This PR fixes a bug where MissingOptionsException error messages for nested prototypes generated an incomplete path. When an option was missing inside a prototype that was itself nested inside another prototype, the key of the parent prototype was omitted from the error message.

The fix ensures the parent's prototype index is correctly tracked and prepended when resolving nested options, providing a full and accurate path for easier debugging.

Example:

Given the following nested prototype setup:

$resolver->setOptions('connections', static function (OptionsResolver $connResolver) {
    $connResolver->setPrototype(true); // Parent prototype
    // ...
    $connResolver->setOptions('replicas', static function (OptionsResolver $replicaResolver) {
        $replicaResolver->setPrototype(true); // Nested prototype
        $replicaResolver->setRequired(['host']);
    });
});

And this configuration, which is missing a host in main_db's second replica:

$options = [
    'connections' => [
        'main_db' => [
            // ...
            'replicas' => [
                ['host' => 'replica-01.local'],
                [], // Missing 'host' here
            ],
        ],
    ],
];
$resolver->resolve($options);

Before (The Bug):
The exception message was incorrect, missing the main_db key:
The required option "connections[replicas][1][host]" is missing.

After (The Fix):
The exception message is now correct and includes the full path:
The required option "connections[main_db][replicas][1][host]" is missing.

A test case (testNestedPrototypeErrorPathHasFullContext) has been added to cover this scenario and prevent regressions.