CVE-2026-19481: @fastify/busboy vulnerable to Denial of Service via prototype-named multipart part header

### Impact Versions of `@fastify/busboy` from 1.0.0 and prior to 3.2.1 are vulnerable to a Denial of Service. The multipart header parser stores part-header names on a plain JavaScript object, so a part header named `__proto__` or `constructor` resolves to an inherited value that is not an array, and the parser throws `TypeError: this.header[h].push is not a function`. Through the documented `req.pipe(busboy)` integration this surfaces as an `error` event, while direct `write()`/`end()` usage throws synchronously and can terminate the Node.js process if uncaught. The parser runs before application middleware, so any unauthenticated client that can submit multipart/form-data is affected. ### Patches Fixed in version 3.2.1. ### Workarounds Attach an `error` listener to the Busboy stream so the parser failure is handled rather than crashing the process, and wrap direct `write()`/`end()` calls in a try/catch. Upgrading to 3.2.1 removes the failure entirely.