Fix code review comments: improve docblock and JSON validation

Co-authored-by: Thavarshan <10804999+Thavarshan@users.noreply.github.com>

Author: Copilot

src/Fetch/Concerns/PerformsHttpRequests.php

@@ -255,7 +255,7 @@ protected function executeWithMiddleware(
                 $contentType = $request->getHeaderLine('Content-Type');
                 if (str_contains($contentType, 'application/json')) {
                     $decoded = json_decode($bodyContents, true);
-                    if ($decoded !== null) {
+                    if (json_last_error() === JSON_ERROR_NONE) {
                         $options['json'] = $decoded;
                         unset($options['body']);
                     } else {

src/Fetch/Interfaces/MiddlewareInterface.php

@@ -20,7 +20,7 @@ interface MiddlewareInterface
      *
      * @param  RequestInterface  $request  The HTTP request to process
      * @param  callable  $next  The next middleware or final handler
-     * @return Response|PromiseInterface<Response> The response or promise
+     * @return \Fetch\Interfaces\Response|PromiseInterface<\Fetch\Interfaces\Response> The response or promise
      */
     public function handle(RequestInterface $request, callable $next): Response|PromiseInterface;
 }