fix(agent): Handle expired setup tokens without refresh token

Add validation to check for refresh token availability before attempting
token refresh. Setup tokens that expire without a refresh token now
return a clear error message instead of attempting an invalid refresh.

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

src/agent/accounts/usage.ts

@@ -222,7 +222,7 @@ export async function checkAccountHealth(
   try {
     // Use credential manager if provided, otherwise fall back to direct functions
     let accessToken: string;
-    let currentExpiresAt: number;
+    let currentExpiresAt: number | null;
     let subscriptionType: string | null = null;
 
     if (credentialManager) {
@@ -293,7 +293,7 @@ export async function checkAccountHealth(
       ...base,
       credentialsValid: true,
       tokenValid: true,
-      tokenExpiresAt: new Date(currentExpiresAt).toISOString(),
+      tokenExpiresAt: currentExpiresAt ? new Date(currentExpiresAt).toISOString() : null,
       subscriptionType,
       usage,
     };