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

Updated checkAccountHealth to handle setup tokens with null expiresAt:
- Changed currentExpiresAt type from number to number | null
- Use conditional for tokenExpiresAt ISO string conversion

This completes the OAuth setup token support across all credential
reading and validation functions.

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

src/agent/accounts/usage.ts

@@ -327,6 +327,11 @@ export async function ensureAccountCredentials(configDir: string): Promise<boole
     return true;
   }
 
+  if (!credentials.refreshToken) {
+    log.error({ configDir }, 'setup token expired, no refresh token available');
+    return false;
+  }
+
   log.info({ configDir }, 'credentials expired, refreshing before spawn');
   const refreshed = await refreshToken(credentials.refreshToken);
   if (!refreshed) {