[12.x] Fix Concurrency::run to preserve callback result order

[chaker2710]

Problem

The current implementation of Concurrency::run() with fork driver does not preserve the original order of callbacks when returning results. This leads to incorrect variable assignment when destructuring the return value, as shown in the documentation example:

[$userCount, $orderCount] = Concurrency::run([
    fn () => DB::table('users')->count(),
    fn () => DB::table('orders')->count(),
]);

If the second callback completes before the first, the results array may be returned in a different order, causing $userCount and $orderCount to be swapped.

Solution

This PR modifies the ForkDriver::run method to associate results with their original keys and return them in the original order after all tasks are completed.

[taylorotwell]

Does this work with associative arrays?

[chaker2710]

Yes, it works.

The destructuring (list unpacking) syntax like:

[$userCount, $orderCount] = Concurrency::run([
    fn () => DB::table('users')->count(),
    fn () => DB::table('orders')->count(),
]);

can only be used with indexed (numerical) arrays.

If we use associative arrays, the syntax must be:

['users' => $userCount, 'orders' => $orderCount] = Concurrency::run([
    'users' => fn () => DB::table('users')->count(),
    'orders' => fn () => DB::table('orders')->count(),
]);

Just wrote a test for this (not committed yet).
If we’re okay with adding spatie/fork as a dev dependency, I’ll push it.