fail SCRAM exchange on invalid ServerSignature

[madib06ops]

1. processScramAuthenticationInfo checks the SCRAM ServerSignature from Authentication-Info (and Proxy-Authentication-Info) but on a mismatch only logged a warning, so a response from a peer that never proved knowledge of the shared secret was still delivered as a success, voiding the mutual-authentication guarantee.
2. RFC 7804 section 5 requires the client to treat that as an unsuccessful exchange.

Abort the request when the ServerSignature is present but invalid or unparseable, for both the origin and proxy SCRAM paths. A missing header stays warn-only, since it may legitimately arrive in chunked trailers that AHC does not read. Added a regression test that drives a full handshake and returns a corrupted ServerSignature.