[11.x] Fix/Improve Resend transport response handling

[markovic-nikola]

Purpose of this PR

This small PR improves the error messages displayed when the Resend API returns an HTTP status code other than 200. The goal is to make errors easier to understand without requiring extensive debugging.

Problem

For example, when the daily quota is reached, the application throws an exception that lacks clarity:
Symfony\Component\Mime\Header\Headers::addTextHeader(): Argument #2 ($value) must be of type string, null given, called in /home/nikola/Projects/project/vendor/symfony/mime/Header/Headers.php on line 154

Solution

With this PR, such cases are handled gracefully, and a clearer error message is provided. For instance, when the daily quota is reached, the error would now read:
Request to Resend API failed. Reason: You have reached your daily email sending quota.

Resend response codes

[jayanratna]

Just wanted to mention that the statusCode is not returned for successful responses, it is only returned for error responses. This would mean that $result['statusCode'] != Response::HTTP_OK would return true as its not apart of the $result.

Tested this logic on https://github.com/resend/resend-laravel with its test suite.

[markovic-nikola]

Hi @jayanratna,
Thanks for mentioning it, I have created another PR to address it.