## Why
Managed networking implicitly selected the elevated Windows sandbox backend,
even when the sandbox was configured to use a restricted token.
## What changed
- Select the Windows sandbox backend solely from `WindowsSandboxLevel`.
- Reject managed networking with a restricted-token sandbox before spawning a
process, since managed networking requires the elevated backend.
- Cover the rejection through sandbox preparation, unified exec, and Windows
sandbox session tests.
GitOrigin-RevId: 308858652d7b629af623d22896dafde3a23d3758