Preserve Windows sandbox proxy settings in guardian sessions (#35036)

## Why

Guardian review commands can run without the proxy-port environment used to
configure the parent session. Reconciling persistent Windows sandbox settings
for those commands can discard the parent's proxy configuration.

## What changed

- Add a session-level Windows sandbox proxy-settings mode and use `Preserve`
  for guardian review sessions while keeping `Reconcile` as the default.
- Carry the mode through unified exec and the exec-server sandbox context to
  Windows process launches.
- In preserve mode, reuse the proxy settings recorded by the existing sandbox
  setup marker.

## Testing

Added coverage for guardian session configuration, exec-server transport, and
preserving an existing setup marker when proxy-port environment variables are
absent.

GitOrigin-RevId: 214655c6e6d97360906431773dc0de0fdda2db6e

codex-rs/file-system/src/lib.rs

@@ -2,6 +2,7 @@ mod find_up;
 
 use bytes::Bytes;
 use codex_protocol::config_types::WindowsSandboxLevel;
+use codex_protocol::config_types::WindowsSandboxProxySettingsMode;
 use codex_protocol::models::ManagedFileSystemPermissions;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::models::SandboxEnforcement;
@@ -291,6 +292,8 @@ pub struct FileSystemSandboxContext {
     pub windows_sandbox_level: WindowsSandboxLevel,
     #[serde(default)]
     pub windows_sandbox_private_desktop: bool,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub windows_sandbox_proxy_settings_mode: Option<WindowsSandboxProxySettingsMode>,
     #[serde(default)]
     pub use_legacy_landlock: bool,
 }
@@ -332,6 +335,7 @@ impl FileSystemSandboxContext {
             workspace_roots,
             windows_sandbox_level: WindowsSandboxLevel::Disabled,
             windows_sandbox_private_desktop: false,
+            windows_sandbox_proxy_settings_mode: None,
             use_legacy_landlock: false,
         }
     }