fix(uploads): preserve Blob files and multipart media types (#2467)

## Summary

- Keep ordinary unnamed Blob uploads in buffered and streaming multipart
requests so public image-edit inputs and masks are never silently
omitted.
- Preserve standard FormData Blob filenames, existing streaming
fallbacks, safe custom filenames, sparse input ordering, and lazy
multipart consumption.
- Retain fetched Response MIME types for public image and
audio-transcription uploads.

## Validation

- Added eight regression-first public-client and multipart tests; all
eight fail on the original code and pass with this change.
- Full handwritten suite: 6,195 passing tests.
- Full generated suite against an isolated owner-verified Steady
instance: 556 passing tests, one pre-existing skip.
- Canonical lint, strict repository typecheck, build, published
TypeScript 4.9 and current TypeScript checks, and publint all pass.
- Packed npm verification passes 275/314 source checks across 1,256
source maps.
- Confirmed the actual built npm client preserves Blob image/mask bytes
and audio/wav Response uploads.