R1:ea6c0c9bdebf

Refine the "Mangled Webserver Response" setup check

Summary:
Ref T13259. In some configurations, making a request to ourselves may return a VPN/Auth response from some LB/appliance layer.

If this response begins or ends with whitespace, we currently detect it as "extra whitespace" instead of "bad response".

Instead, require that the response be nearly correct (valid JSON with some extra whitespace, instead of literally anything with some extra whitespace) to hit this specialized check. If we don't hit…