Cast Content-Length header value to string to avoid psr7 deprecation

guzzlehttp/psr7 2.11 deprecates passing a non-string to
MessageInterface::withHeader(), and 3.0 will require string|string[].
mb_strlen() returns an int, so every request with a body emits:

  Passing int to MessageInterface::withHeader() is deprecated;
  guzzlehttp/psr7 3.0 requires string|string[].

Cast the value to string to silence the deprecation and stay
forward-compatible with psr7 3.0.

Author: iitenkida7

CHANGELOG.md

@@ -6,6 +6,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
+- [Patch] Cast `Content-Length` header value to `string` in `Http::request()` to avoid a `guzzlehttp/psr7` 2.11 deprecation (`Passing int to MessageInterface::withHeader() is deprecated`) and stay compatible with `guzzlehttp/psr7` 3.0
 
 ## v6.1.1 - 2026-03-02
 - [#456](https://github.com/Shopify/shopify-api-php/pull/456) [Patch] Update firebase/php-jwt to ^7.0 to address security vulnerability (GHSA-2x45-7fc3-mxwq)

src/Clients/Http.php

@@ -207,7 +207,7 @@ protected function request(
             $request = $request
                 ->withBody($stream)
                 ->withHeader(HttpHeaders::CONTENT_TYPE, $dataType)
-                ->withHeader(HttpHeaders::CONTENT_LENGTH, mb_strlen($bodyString));
+                ->withHeader(HttpHeaders::CONTENT_LENGTH, (string) mb_strlen($bodyString));
         }
 
         $currentTries = 0;