feat: support restricted ReadOnlyAccess in elevated Windows sandbox (#14610)

## Summary
- support legacy `ReadOnlyAccess::Restricted` on Windows in the elevated
setup/runner backend
- keep the unelevated restricted-token backend on the legacy full-read
model only, and fail closed for restricted read-only policies there
- keep the legacy full-read Windows path unchanged while deriving
narrower read roots only for elevated restricted-read policies
- honor `include_platform_defaults` by adding backend-managed Windows
system roots only when requested, while always keeping helper roots and
the command `cwd` readable
- preserve `workspace-write` semantics by keeping writable roots
readable when restricted read access is in use in the elevated backend
- document the current Windows boundary: legacy `SandboxPolicy` is
supported on both backends, while richer split-only carveouts still fail
closed instead of running with weaker enforcement

## Testing
- `cargo test -p codex-windows-sandbox`
- `cargo check -p codex-windows-sandbox --tests --target
x86_64-pc-windows-msvc`
- `cargo clippy -p codex-windows-sandbox --tests --target
x86_64-pc-windows-msvc -- -D warnings`
- `cargo test -p codex-core windows_restricted_token_`

## Notes
- local `cargo test -p codex-windows-sandbox` on macOS only exercises
the non-Windows stubs; the Windows-targeted compile and clippy runs
provide the local signal, and GitHub Windows CI exercises the runtime
path

codex-rs/windows-sandbox-rs/src/elevated/command_runner_win.rs

@@ -199,11 +199,6 @@ fn spawn_ipc_process(
     );
 
     let policy = parse_policy(&req.policy_json_or_preset).context("parse policy_json_or_preset")?;
-    if !policy.has_full_disk_read_access() {
-        anyhow::bail!(
-            "Restricted read-only access is not yet supported by the Windows sandbox backend"
-        );
-    }
     let mut cap_psids: Vec<*mut c_void> = Vec::new();
     for sid in &req.cap_sids {
         let Some(psid) = (unsafe { convert_string_sid_to_sid(sid) }) else {