fix(server::peers::operator::auth): make ChallengeContext pub for smlang state machine

smlang generates a public state enum whose variants contain ChallengeContext,
requiring the type itself to be fully public. Also tightens the wildcard arm
in client auth to an exhaustive match.

server/crates/arbiter-server/src/peers/client/auth.rs

@@ -298,7 +298,7 @@ where
 
     let signature = expect_message(transport, |req: Inbound| match req {
         Inbound::AuthChallengeSolution { signature } => Some(signature),
-        _ => None,
+        Inbound::AuthChallengeRequest { .. } => None,
     })
     .await
     .map_err(|e| {