Complete return types in HTTP client

[Strift]

The HTTP Client and Contract interface need more specific return types and PHPDoc annotations.

Files & Actions:

1. src/Http/Client.php

   • execute(RequestInterface $request, array $headers = []): Add return type : mixed or PHPDoc @return array|null.
   • parseResponse(ResponseInterface $response): Add return type : mixed or PHPDoc @return array|null.
   • buildQueryString(array $queryParams = []): Add PHPDoc @param array<string, mixed> $queryParams.

2. src/Contracts/Http.php

   • Update get (and similar methods) to include PHPDoc:
     @param array<string, scalar|list<scalar>> $query