Closes #859

Summary

• remove the statically analyzable @vercel/blob/client import path from useMultipartUpload
• load Vercel Blob client at runtime through a lazy importer helper
• throw a clear actionable error when Vercel Blob client is missing:
  • @vercel/blob is required to use useMultipartUpload with Vercel Blob
  • pnpm add @vercel/blob
• add focused tests for the helper behavior (module present, module missing, missing upload export)

Reproduction (Cloudflare-oriented)

Baseline repro app (@nuxthub/core@0.10.7, hub.blob: true, nitro.preset: 'cloudflare_module', useMultipartUpload referenced, no @vercel/blob):

pnpm nuxt build

Before this fix:

• build fails with Rollup failed to resolve import "@vercel/blob/client" from useMultipartUpload.mjs

After this fix (installing this patched package into the same repro app):

• pnpm nuxt build completes successfully

Tests

• pnpm eslint src/blob/runtime/app/composables/useMultipartUpload.ts test/useMultipartUpload.vercel.test.ts
• pnpm vitest test/useMultipartUpload.vercel.test.ts
• pnpm vitest test/blob.test.ts
• external repro check:
  • before patch: Cloudflare preset build fails with unresolved import
  • after patch: Cloudflare preset build succeeds without @vercel/blob