Require the elevated Windows sandbox for network proxies (#32857)

## Why

Network proxy enforcement on Windows requires the elevated sandbox backend. Silently selecting that backend for an unelevated configuration makes the effective sandbox differ from the configured mode.

## What changed

- Reject enabled network proxy configurations on Windows unless managed requirements allow only the elevated sandbox and `windows.sandbox` resolves to `"elevated"`.
- Preserve the configured sandbox backend during execution and validate proxy compatibility at config loading, sandbox setup, and process launch boundaries.
- Defer this validation during cloud-config bootstrap so authoritative managed requirements can be loaded first.
- Do not start or expose disabled Windows proxies, and render transitions to disabled networking as `<network enabled="false"></network>`.

## Testing

- Cover Windows requirement and sandbox compatibility matrices, bootstrap behavior, backend selection, disabled proxy handling, and environment-context updates.

GitOrigin-RevId: bfa1dda98594e0db61883f8e7f65bd560e3453e6

codex-rs/core/src/session/session.rs

@@ -963,7 +963,12 @@ impl Session {
                         )
                     });
             let (network_proxy, session_network_proxy) =
-                if let Some(spec) = config.permissions.network.as_ref() {
+                if let Some(spec) = config
+                    .permissions
+                    .network
+                    .as_ref()
+                    .filter(|spec| !cfg!(target_os = "windows") || spec.enabled())
+                {
                     let current_exec_policy = exec_policy.current();
                     let (network_proxy, session_network_proxy) = Self::start_managed_network_proxy(
                         spec,