Fix handling of 422 responses where errors is a string

[GrahamCampbell]

GitHub sometimes returns a 422 response in which the errors property is a plain string rather than an array of error objects, for example when creating a commit status for a commit that cannot be found. The exception thrower currently iterates over that property unconditionally, so instead of throwing a ValidationFailedException carrying the message from GitHub, it triggers a PHP warning about passing a string to foreach, which under most frameworks surfaces as an ErrorException and hides the real error. This change casts the property to an array before iterating, so a string is reported through the existing string handling branch, and adds a test case covering that response shape.

[acrobat]

Thank you @GrahamCampbell

[acrobat]

Released https://github.com/KnpLabs/php-github-api/releases/tag/v3.16.1