feat(provider): harden Provider API transport — split usage chunk, non-image file rejection, dead export cleanup (issue #58)

Close the Provider API review gaps:
- Hold the finish part until the SSE body is fully drained so an OpenAI
  trailing usage-only chunk (choices:[]) is consumed and the finish part
  carries real token counts/cost instead of zeroed usage. Preserve a
  non-stop finish_reason (e.g. length) across the split chunk.
- Reject non-image file parts on the Provider API codecs (audio/*,
  application/*, or a file with no verifiable image type) with a clear
  role-aware error instead of silently base64-encoding them; image/*
  files still forward.
- Remove the dead/speculative alias exports from converters.ts and
  stream.ts, and the unused DEFAULT_BASE_URL constant.