[codex] Preserve proxy state for filesystem sandbox helpers (#29671)

## Why

Filesystem helpers intentionally run with a minimal environment that
excludes proxy variables. After filesystem operations started using the
Windows sandbox wrapper, the wrapper derived an empty proxy
configuration from that helper environment and compared it with the
persistent sandbox setup marker. When the marker contained proxy ports,
every filesystem operation appeared to require a firewall update, which
could launch elevated setup, show a UAC or loader dialog, and fail
operations such as `apply_patch` with error 1223.

Filesystem helpers do not use network access, so they should preserve
the proxy/firewall state established by normal sandboxed process
launches.

## What changed

- Add an explicit Windows sandbox proxy-settings mode for reconciling or
preserving persistent proxy state.
- Use preserve mode for filesystem helpers while normal process launches
continue to reconcile proxy settings from their environment.
- Carry the selected proxy state consistently through setup validation,
elevated setup, and non-elevated ACL refreshes.
- Cover wrapper argument propagation and marker-derived proxy
preservation.

## Validation

- `cargo build -p codex-cli --bin codex`
- `just test -p codex-windows-sandbox
preserving_proxy_settings_uses_the_existing_marker`
- `just test -p codex-windows-sandbox windows_wrapper_args_round_trip`
- `just test -p codex-windows-sandbox
setup_request_prefers_explicit_proxy_settings`
- `just test -p codex-sandboxing transform_for_direct_spawn_windows`
- `just test -p codex-exec-server fs_sandbox::tests`
- Ran the same sandboxed `fs/writeFile` reproduction against published
`0.142.0-alpha.6` and the new CLI. The published CLI launched elevated
setup and failed with `ShellExecuteExW ... 1223`; the new CLI completed
without elevation.

Related to #28359.

codex-rs/windows-sandbox-rs/src/unified_exec/backends/elevated.rs

@@ -56,6 +56,7 @@ pub(crate) async fn spawn_windows_sandbox_session_elevated_for_permission_profil
     cwd: &Path,
     mut env_map: HashMap<String, String>,
     proxy_enforced: bool,
+    proxy_settings_mode: crate::WindowsSandboxProxySettingsMode,
     timeout_ms: Option<u64>,
     read_roots_override: Option<&[PathBuf]>,
     read_roots_include_platform_defaults: bool,
@@ -91,6 +92,7 @@ pub(crate) async fn spawn_windows_sandbox_session_elevated_for_permission_profil
         &deny_read_paths_override,
         &deny_write_paths_override,
         proxy_enforced,
+        proxy_settings_mode,
     )?;
 
     let spawn_request = SpawnRequest {
@@ -133,6 +135,7 @@ pub(crate) async fn spawn_windows_sandbox_session_elevated_for_permission_profil
                 &deny_read_paths_override,
                 &deny_write_paths_override,
                 /*proxy_enforced*/ false,
+                proxy_settings_mode,
             )?;
             spawn_runner_transport_task(
                 codex_home,

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

@@ -31,6 +31,7 @@ pub struct WindowsSandboxSessionRequest<'a> {
     pub env_map: HashMap<String, String>,
     pub windows_sandbox_level: WindowsSandboxLevel,
     pub proxy_enforced: bool,
+    pub proxy_settings_mode: crate::WindowsSandboxProxySettingsMode,
     pub timeout_ms: Option<u64>,
     pub read_roots_override: Option<&'a [PathBuf]>,
     pub read_roots_include_platform_defaults: bool,
@@ -48,7 +49,7 @@ pub async fn spawn_windows_sandbox_session_for_level(
     if request.proxy_enforced
         || matches!(request.windows_sandbox_level, WindowsSandboxLevel::Elevated)
     {
-        spawn_windows_sandbox_session_elevated_for_permission_profile(
+        backends::elevated::spawn_windows_sandbox_session_elevated_for_permission_profile(
             request.permission_profile,
             request.workspace_roots,
             request.codex_home,
@@ -56,6 +57,7 @@ pub async fn spawn_windows_sandbox_session_for_level(
             request.cwd,
             request.env_map,
             request.proxy_enforced,
+            request.proxy_settings_mode,
             request.timeout_ms,
             request.read_roots_override,
             request.read_roots_include_platform_defaults,
@@ -145,6 +147,7 @@ pub async fn spawn_windows_sandbox_session_elevated_for_permission_profile(
         cwd,
         env_map,
         proxy_enforced,
+        crate::WindowsSandboxProxySettingsMode::Reconcile,
         timeout_ms,
         read_roots_override,
         read_roots_include_platform_defaults,