An RFC 7807 problem detail returned by Glean.
Glean's Platform API defines ProblemDetail as its error body, and the other
APIs return it for most 4xx and 5xx responses. This is the transport-level
normalisation of that body: it is parsed with no schema lookup so that error
handling works even for endpoints whose responses are not in the spec.
raw always holds the decoded body exactly as received, so nothing is lost
when Glean adds fields.