## Why
Sandboxes that enforce endpoint access directly need dedicated loopback proxy ports instead of shared SID-attributed ingress.
## What changed
Expose `ManagedProxyRouting` through `NetworkProxyBuilder::managed_proxy_routing`. Selecting `DedicatedListeners` reserves per-proxy loopback listeners on Windows as well as other platforms. Keep `SharedIngress` as the default and include the routing mode in proxy equality.
## Testing
Add regression coverage for distinct loopback endpoints, sandbox port metadata, and HTTP and SOCKS allow/deny policy enforcement. On Windows, verify that dedicated routing requires no restricting SID and omits the shared-ingress proxy-port environment variable.
GitOrigin-RevId: e198891bef1d089f9492d2982505a2d6bb002a74