Fix: Add JSON_UNESCAPED_UNICODE to preserve UTF-8 characters

[aleksandar-mitrev]

What:

• Bug Fix
• New Feature

Description:

This PR fixes a bug where non-ASCII characters (e.g., German umlauts like ü and Ö) were being incorrectly escaped in the JSON request body (e.g., to \u00fc and \u00d6).

Related:

How to Test

1. Create a request with parameters containing special UTF-8 characters. For example:

   $client->chat()->create([
       'model' => 'gpt-4o',
       'messages' => [
           ['role' => 'user', 'content' => 'Was bedeuten die Umlaute ü und Ö?'],
       ],
   ]);

Intercept the request body before it is sent.

Before this fix: The body would contain "... \u00fc und \u00d6".

After this fix: The body now correctly contains "... ü und Ö".

[iBotPeaches]

This makes sense to me. I'll do some quick tests with OpenAI with escaped and unescaped to double check how AI handles the difference.

[iBotPeaches]

thanks! my tests worked