add toHaveMethodWithReturnType arch expectation

[damiogunsiji]

Adds toHaveMethodWithReturnType to check a method's return type.

// exact match - string must match the return type exactly
arch('services return correct types')
    ->expect('App\Services\PaymentService')
    ->toHaveMethodWithReturnType('process', 'array');

// subset match - checks all given types are present in the union
arch('repositories can return null')
    ->expect('App\Repositories\UserRepository')
    ->toHaveMethodWithReturnType('find', ['User', 'null']);  // matches ?User or User|null

// negated - passes when not all of the given types are present
arch('repositories not returning strings')
    ->expect('App\Repositories\UserRepository')
    ->not->toHaveMethodWithReturnType('find', ['string']);

// show() returns ?array (array|null)
arch('show does not return array, string and null')
    ->expect('App\Models\User')
    ->not->toHaveMethodWithReturnType('show', ['array', 'string', 'null']); // passes (string missing)

arch('show does not return array and null')
    ->expect('App\Models\User')
    ->not->toHaveMethodWithReturnType('show', ['array', 'null']); // fails

arch('show does not return array')
    ->expect('App\Models\User')
    ->not->toHaveMethodWithReturnType('show', ['array']); // fails

When $returnType is a string it matches exactly (e.g. 'array' only matches array). When an array it checks the method's return type contains all the given types (e.g. ['User', 'null'] matches ?User or User|null). PHP's ?Type shorthand is normalized so ?array is treated as ['array', 'null']. The not modifier inverts the assertion — it passes only when at least one of the given types is missing from the method's return type.