fix(smithy-client): decorateServiceException not to overwrite modeled members

Author: AllanZhengYP

packages/smithy-client/src/exceptions.ts

@@ -49,7 +49,7 @@ export const decorateServiceException = <E extends ServiceException>(
     .filter(([, v]) => v !== undefined)
     .forEach(([k, v]) => {
       // @ts-ignore assign unmodeled keys
-      exception[k] = v;
+      exception[k] = exception[k] ?? v;
     });
   // load error message from possible locations
   // @ts-expect-error message could exist in Message key.