Prepare upload

Returns the key an upload should use for the given file — unchanged when free, numerically suffixed ("a.pdf" → "a (2).pdf") when the name is already taken — plus a V4 signed PUT URL for exactly that key. PUT the file body to upload_url sending upload_headers verbatim: they carry an if-generation-match precondition (412 if the object appeared meanwhile — re-resolve and retry) and a content-length-range that GCS enforces on the actual received bytes (400 when over max_size_bytes). Called once per file right before its upload. Refused with 409 inside folders synced from an external data source.