Add a multipart encoder to GutenbergKitHTTP and delete the duplicated multipartBodyStream

Summary

GutenbergKitHTTP has a multipart parser (MultipartPart.parse / parseChunked) but no encoder. The only multipart-building code is MediaUploadServer.multipartBodyStream, which is file-only, hand-rolled, and duplicates RequestBody.makePipedFileSliceStream nearly verbatim — its own doc comment notes it uses "the same pattern as" that helper.

Surfaced during the #357 upload-relay review.

Proposal

Add the encode counterpart to GutenbergKitHTTP: a multipart encoder that takes [MultipartPart] (+ boundary) and streams each part's RequestBody through the same bound-stream primitive makePipedFileSliceStream already uses.

Then:

• The media-upload re-encode path builds [processed-file part] + [original non-file parts] and calls the encoder, instead of hand-writing the multipart.
• Delete MediaUploadServer.multipartBodyStream / writeAll.
• GutenbergKitHTTP gains a symmetric parse ⇄ encode pair.

Context

Split out of #357. The immediate need there — forwarding post / additionalData through the re-encode path — is handled inline via a preamble extension of multipartBodyStream. This issue tracks the cleaner refactor that also removes the duplication.

Files

• ios/Sources/GutenbergKitHTTP/MultipartPart.swift — parser
• ios/Sources/GutenbergKitHTTP/RequestBody.swift — makePipedFileSliceStream
• ios/Sources/GutenbergKit/Sources/Media/MediaUploadServer.swift — multipartBodyStream (to delete)

Start with ios/Sources/GutenbergKitHTTP/MultipartPart.swift and RequestBody.swift, comparing the parser with makePipedFileSliceStream. Then inspect MediaUploadServer.swift to trace multipartBodyStream and writeAll. Done means the encoder streams all MultipartPart values, the media re-encode path uses it, and the duplicated helpers are deleted without losing non-file parts.