* fix(gateway): offload gateway upload file IO
Move Gateway upload router filesystem work off the asyncio event loop by
using a dedicated ContextVar-preserving file IO executor. Use async sandbox
acquisition for non-mounted sandbox uploads and offload remote sandbox sync
together with host file reads.
Add blocking-IO regression coverage for upload, list, delete, and remote
sandbox sync paths.
* fix(gateway): align file IO worker env var prefix
Rename the file IO executor worker-count environment variable from
DEERFLOW_FILE_IO_WORKERS to DEER_FLOW_FILE_IO_WORKERS to match the repo's
existing runtime configuration prefix convention.
