fix(agent): Add refresh token validation before token refresh

Check for refresh token availability before attempting credential refresh.
Setup tokens that expire without a refresh token now return a clear error
instead of attempting an invalid refresh operation.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/agent/mock-manager.ts

@@ -474,9 +474,9 @@ export class MockAgentManager implements AgentManager {
     if (!record) {
       throw new Error(`Agent '${agentId}' not found`);
     }
-    // In mock, we just mark it as dismissed in memory
-    // Real implementation would update database
-    record.info.updatedAt = new Date();
+    const now = new Date();
+    record.info.userDismissedAt = now;
+    record.info.updatedAt = now;
   }
 
   /**