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/windows-sandbox-rs/src/unified_exec/mod.rs

@@ -10,6 +10,7 @@
 mod backends;
 
 use anyhow::Result;
+use anyhow::bail;
 use codex_protocol::config_types::WindowsSandboxLevel;
 use codex_protocol::models::PermissionProfile;
 use codex_utils_absolute_path::AbsolutePathBuf;
@@ -47,8 +48,12 @@ pub async fn spawn_windows_sandbox_session_for_level(
     request: WindowsSandboxSessionRequest<'_>,
 ) -> Result<SpawnedProcess> {
     if request.proxy_enforced
-        || matches!(request.windows_sandbox_level, WindowsSandboxLevel::Elevated)
+        && !matches!(request.windows_sandbox_level, WindowsSandboxLevel::Elevated)
     {
+        bail!("network proxy enforcement requires the elevated Windows sandbox backend");
+    }
+
+    if matches!(request.windows_sandbox_level, WindowsSandboxLevel::Elevated) {
         backends::elevated::spawn_windows_sandbox_session_elevated_for_permission_profile(
             request.permission_profile,
             request.workspace_roots,