fix: use stage-appropriate message in ManagedRelayDpopProofCreationError

When stage is 'load-key', the message now says 'Could not load the relay
DPoP key' instead of incorrectly saying 'Could not create the relay DPoP
proof', matching the actual failure phase.

Author: cursoragent

packages/client-runtime/src/relay/managedRelay.ts

@@ -71,7 +71,9 @@ export class ManagedRelayDpopProofCreationError extends Schema.TaggedErrorClass<
   },
 ) {
   override get message(): string {
-    return `Could not create the relay DPoP proof for ${this.method} ${this.url}.`;
+    return this.stage === "load-key"
+      ? `Could not load the relay DPoP key for ${this.method} ${this.url}.`
+      : `Could not create the relay DPoP proof for ${this.method} ${this.url}.`;
   }
 }