chore: refine oauth token expiry handling

Co-authored-by: samanhappy <2755122+samanhappy@users.noreply.github.com>

src/services/mcpOAuthProvider.ts

@@ -376,9 +376,6 @@ export class MCPHubOAuthProvider implements OAuthClientProvider {
       const parsed = Date.parse(accessTokenExpiresAt);
       return Number.isNaN(parsed) ? undefined : parsed;
     }
-    if (accessTokenExpiresAt instanceof Date) {
-      return accessTokenExpiresAt.getTime();
-    }
     return undefined;
   }