## What changed
Move proxy configuration and routing selection into `prepare_managed_network`. Keep native Windows launches direct by stripping managed proxy variables before credential brokering, and select dedicated listeners only for Windows MXC launches.
Consolidate managed network context filtering in `SandboxManager` and simplify proxy preparation for Linux and MXC while preserving the macOS Seatbelt context.
## Testing
Add Windows regression coverage for native launches with both disabled and enabled proxy configurations, verifying that managed proxy variables are removed while caller environment values and API credentials are preserved.
GitOrigin-RevId: 29fe1fb02afb574b4108032b7e8028d19d8270b4