fix(agent): Handle optional OAuth fields in usage.ts credential reader

Make refreshToken and expiresAt optional in usage credential validation.
Aligns with changes in default-credential-manager.ts.

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

src/agent/accounts/usage.ts

@@ -252,6 +252,14 @@ export async function checkAccountHealth(
       subscriptionType = credentials.subscriptionType;
 
       if (isTokenExpired(credentials)) {
+        if (!credentials.refreshToken) {
+          log.warn({ accountId: account.id }, 'setup token expired, no refresh token');
+          return {
+            ...base,
+            credentialsValid: true,
+            error: 'Setup token expired, no refresh token available',
+          };
+        }
         log.info({ accountId: account.id, email: account.email }, 'token expired, refreshing');
         const refreshed = await refreshToken(credentials.refreshToken);
         if (!refreshed) {