Add hidden Windows sandbox wrapper entrypoint (#28358)

## Why

This is the second PR in the Windows fs-helper sandbox stack. The
fs-helper path needs a Windows sandbox launcher that has the same
argv-shaped contract as macOS `sandbox-exec` and `codex-linux-sandbox`,
but this PR only introduces that hidden launcher. It does not route
fs-helper through it yet.

The hidden launcher still needs to be policy-complete before later
direct-spawn callers use it. In particular, it has to carry the same
Windows sandbox policy details that the existing spawn paths already
understand: proxy enforcement, read/write root overrides, and
deny-read/deny-write overrides.

## What Changed

- Added the hidden `codex.exe --run-as-windows-sandbox` arg1 dispatch
path.
- Added `windows-sandbox-rs/src/wrapper.rs`, which parses the wrapper
argv, launches the requested command through the shared Windows sandbox
session runner from PR1, and forwards stdio.
- Added `create_windows_sandbox_command_args_for_permission_profile()`
so later direct-spawn callers can build the wrapper argv consistently.
- Made the wrapper argv round-trip the full Windows sandbox policy
surface it needs later: workspace roots, environment, permission
profile, sandbox level, private desktop, proxy enforcement, read/write
root overrides, and deny-read/deny-write overrides.
- Carried `proxy_enforced` through the shared Windows session request so
proxy-managed executions continue to use the offline/elevated sandbox
identity.
- Added wrapper argument round-trip coverage for the full policy fields.

## Verification

- `just test -p codex-windows-sandbox windows_wrapper_args_round_trip`
- `just test -p codex-arg0`
- `just test -p codex-core exec::tests::windows_`
- `just fix -p codex-windows-sandbox -p codex-core -p codex-cli`

Local note: the full `just fmt` command still fails on this workstation
in non-Rust formatter setup (`uv` cache access denied and missing
`dotslash`/buildifier), but the Rust formatter phase completed.

codex-rs/cli/src/debug_sandbox.rs

@@ -382,6 +382,7 @@ async fn run_command_under_windows_session(
         cwd: cwd.as_path(),
         env_map: env,
         windows_sandbox_level: WindowsSandboxLevel::from_config(config),
+        proxy_enforced: false,
         timeout_ms: None,
         read_roots_override: None,
         read_roots_include_platform_defaults: false,