vitest's toMatchObject doesn't accept a type parameter; tests passed
under vitest (esbuild transpile) but svelte-check rejected. Plan-spec
bug introduced in 4d1a7f8; fixing here so check is clean before Task 6.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Wraps fetch, parses JSON, and throws ApiError{code,message,status}
on non-2xx. Degrades to {code:'unknown'} when the error body is not
the expected envelope shape.
bvandeusen