## Why
Streaming reads previously rejected requests that used a platform filesystem
sandbox.
## What changed
- Open streamed files in the sandbox helper and return the open file to the
exec-server by passing a file descriptor on Unix or duplicating a file handle
on Windows.
- Advertise support through the `sandboxedFileStreaming` environment capability.
- Preserve close-on-exec behavior for transferred descriptors, including the
required inherited-descriptor cleanup on macOS.
## Testing
- Cover bounded sandboxed streams, continued reads after path replacement, and
rejection of symlink escapes outside readable roots.
GitOrigin-RevId: 677b2444b74e834b78b87a8554bc119c1c6e08b2