fix(cli): preserve signed upload content-type headers

[user-github-me]

The direct-upload flow adds a lowercase content-type default before spreading the server’s signed headers. If the API returns Content-Type or another casing, fetch combines both values; a real HTTP request receives application/zip, application/zip, changing the header that the presigned request requires.

Add the ZIP default only when the normalized upload headers contain no content-type key, comparing names case-insensitively. Preserve the server’s values and casing and keep the existing handling of other headers.

Validation: 113 cloud/client and cloud-command tests pass. Five real HTTP upload cases cover title, uppercase, mixed, lowercase, and absent content-type keys; the first three fail on current main. The tests verify the received header value, one header entry, unchanged request bytes, and upload completion. A separate Node 22 HTTP probe confirms the duplicate value before and a single value after. CLI typecheck, repository lint, formatting, and test reachability pass.

[jrusso1020]

Thanks. We checked the server side, and the direct-upload endpoint always returns upload_headers with a lowercase content-type key, so the spread already overrides the default and no duplicate value is sent. Since the mixed-case scenario can't occur with the current API, we don't think this change is needed. — Rames