fix(blob): validate multipart upload body

[onmax]

Fixes #762

Problem

handleMultipartUpload throws cryptic 500 error "cannot read properties of undefined (reading 'type')" when request body is empty/invalid.

Changes

1. Fix: Added body validation in vercel-blob.ts before passing to Vercel's handleUpload
2. Docs: Clarified that Vercel Blob uses [...pathname] route pattern (no [action])

Reproduction

git clone --depth 1 --filter=blob:none --sparse https://github.com/onmax/repros.git
cd repros && git sparse-checkout set nuxthub-762
cd nuxthub-762 && pnpm i && pnpm dev

Trigger error (in another terminal):

curl -X POST http://localhost:3005/api/blob/multipart/test.pdf -H "Content-Type: application/json"

Before: 500 Cannot read properties of undefined (reading 'type')

Verify fix

cd .. && git sparse-checkout add nuxthub-762-fixed
cd nuxthub-762-fixed && pnpm i && pnpm dev

Same curl command now returns:

After: 400 Invalid multipart upload request body

The -fixed folder includes a pnpm patch with the fix.

[vercel]

@onmax is attempting to deploy a commit to the NuxtLabs Team on Vercel.

A member of the Team first needs to authorize it.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxthub/core@763

commit: 0b01303