api: throw when the JSON request body cannot be encoded

[robertstaddon]

Fixes #856

Summary

MC4WP_API_V3_Client::request() set the HTTP body to the raw return value of json_encode(). When the subscribe payload cannot be encoded, that return value is boolean false. wp_remote_request() passes it to WpOrg\Requests\Transport\Curl::request(), which requires array|string and throws WpOrg\Requests\Exception\InvalidArgument.

That exception is not an MC4WP_API_Exception, so list_subscribe() does not catch it and the form POST fatals during init. No other plugin is required. mc4wp_sanitize_deep() keeps invalid UTF-8, and its substr($value, 0, 1024) cut can split a multibyte character. Either value reaches this method on a stock install.

This encodes POST, PUT, and PATCH bodies with wp_json_encode(), which repairs invalid UTF-8 before encoding. If the result is still not a string, it throws MC4WP_API_Exception and does not call wp_remote_request(). After mc4wp_http_request_args, a body that is neither a string nor an array throws the same exception. list_subscribe() already turns that into a form error.

Test plan

• Submit a published form with a valid email and invalid UTF-8 in a text field (for example the bytes C3 28), with the honeypot empty and a normal User-Agent. The page should show the form error instead of a fatal.
• Submit the same form with a normal name. The subscriber should still be sent to Mailchimp.
• Confirm a mc4wp_http_request_args callback that leaves body as a JSON string still sends the request.

Made with Cursor

[dannyvankooten]

Great catch, Thanks for the PR @robertstaddon!