Rename host instruction payload to `Instructions` (#40709)

## What changed

Rename the `UserInstructions` type to `Instructions` across the extension API,
its core API re-export, and all consumers. This keeps loading behavior and the
`LoadedUserInstructions` provider interface unchanged.

GitOrigin-RevId: 1ac236b34c31718e7aa38ff5eae5e9b179aeedd8