multiparty: Denial of Service via Prototype Pollution leads to Uncaught Exception

### Impact multiparty@4.2.3 and lower versions are vulnerable to denial of service via uncaught exception. By sending a `multipart/form-data` request with a field name that collides with an inherited `Object.prototype` property (e.g., `__proto__`, `constructor`, `toString`), the parser invokes `.push()` on the inherited prototype value rather than an array, throwing a `TypeError` that propagates as an uncaught exception and crashes the process. Any service accepting multipart uploads via multiparty is affected. ### Patches Users should upgrade to multiparty@4.3.0 or higher. ### Workarounds None.