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/exec.rs

@@ -17,6 +17,7 @@ use tokio::io::BufReader;
 use tokio::process::Child;
 use tokio_util::sync::CancellationToken;
 
+use crate::config::validate_windows_sandbox_network_proxy_compatibility;
 use crate::sandboxing::ExecOptions;
 use crate::sandboxing::ExecRequest;
 use crate::sandboxing::SandboxPermissions;
@@ -411,10 +412,13 @@ pub fn build_exec_request(
             )
         })
         .map_err(CodexErr::from)?;
-    let use_windows_elevated_backend = windows_sandbox_uses_elevated_backend(
+    validate_windows_sandbox_network_proxy_compatibility(
         exec_req.windows_sandbox_level,
         exec_req.network.is_some(),
-    );
+    )
+    .map_err(CodexErr::Io)?;
+    let use_windows_elevated_backend =
+        windows_sandbox_uses_elevated_backend(exec_req.windows_sandbox_level);
     exec_req.windows_sandbox_filesystem_overrides = if use_windows_elevated_backend {
         resolve_windows_elevated_filesystem_overrides(
             exec_req.sandbox,
@@ -658,7 +662,9 @@ async fn exec_windows_sandbox(
     let command_path = command.first().cloned();
     let sandbox_level = windows_sandbox_level;
     let proxy_enforced = network.is_some();
-    let use_elevated = windows_sandbox_uses_elevated_backend(sandbox_level, proxy_enforced);
+    validate_windows_sandbox_network_proxy_compatibility(sandbox_level, proxy_enforced)
+        .map_err(CodexErr::Io)?;
+    let use_elevated = windows_sandbox_uses_elevated_backend(sandbox_level);
     let additional_deny_write_paths = windows_sandbox_filesystem_overrides
         .map(|overrides| overrides.additional_deny_write_paths.clone())
         .unwrap_or_default();