FST_FILES_LIMIT not being raised when files limit is exceeded

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the bug has not already been reported

### Fastify version

5.12.0

### Plugin version

10.1.1

### Node.js version

26.7

### Operating system

macOS

### Operating system version (i.e. 20.04, 11.3, 10)

Tahoe 26.6.2

### Description

On `@fastify/multipart@10.1.1`, when a request exceeds `limits.files` and parts are consumed with `part.toBuffer()`, we're seeing:

- `code: 'ERR_STREAM_PREMATURE_CLOSE'`
- `message: 'Premature close'`

instead of:

- `FilesLimitError` / `code: 'FST_FILES_LIMIT'` / `statusCode: 413`

This worked on 10.1.0.

### Link to code that reproduces the bug

https://github.com/fastify/fastify-multipart/pull/641

### Expected Behavior

I expect to see FST_FILES_LIMIT if the file limit is exceeded.

Start with the reproducer linked in the issue, https://github.com/fastify/fastify-multipart/pull/641, using @fastify/multipart 10.1.1 and the stated Node.js environment. Exercise requests that exceed limits.files while consuming parts with part.toBuffer(), then verify that the result is FilesLimitError with code FST_FILES_LIMIT and statusCode 413 rather than ERR_STREAM_PREMATURE_CLOSE.

Written by the indexing model from the issue text.