#4450 GCP codegen: generate media upload/download from Discovery mediaUpload and supportsMediaDownload

The GCP codegen pipeline (codegen/gcp/pipeline.ts, extensionModelGenerator.ts, libGenerator.ts) ignores the Discovery document fields supportsMediaUpload, mediaUpload (simple and resumable protocols), supportsMediaDownload and useMediaDownloadService. Every generated GCP model therefore sends JSON-only bodies and parses JSON-only responses, so no media-capable method (storage.objects insert/get, youtube uploads, drive.files create/update/get/export, and others) can send or receive file contents.

A concrete symptom: the generated drive files export method calls files.export, which returns raw exported bytes, and then parses them as JSON.

Proposal: emit generic media methods from those Discovery fields: multipart and resumable upload, plus alt=media streamed download into a file spec. This needs a binary-capable authorized request helper in the shared lib, and changing _lib/gcp.ts force-bumps every GCP model, so it needs a design pass on blast radius.

Context: swamp-club#4449 addresses Drive Files alone through a codegen enrichment; this issue tracks the generic, provider-wide version.