The Rust HyperServer already parses multipart/form-data, but the test-side
RequestBuilder could only build json/form/raw bodies — so upload routes
(request.file()) had no ergonomic way to be integration-tested. Add
.field(name, value) and .file(field, Buffer|string, { filename, contentType })
mirroring Adonis/japa's api-client, encoding a RFC 7578 multipart/form-data
body with a generated boundary at send time.

Unit tests cover the encoding; an integration test feeds a builder-produced
body to the real HyperServer and asserts req.multipart round-trips fields +
file bytes intact. No Rust change — the receive path was already complete.