Enable --deny-warnings for cargo shear (#21616)

## Summary

In https://github.com/openai/codex/pull/21584, we disabled doctests for
crates that lack any doctests. We can enforce that property via `cargo
shear --deny-warnings`: crates that lack doctests will be flagged if
doctests are enabled, and crates with doctests will be flagged if
doctests are disabled.

A few additional notes:

- By adding `--deny-warnings`, `cargo shear` also flagged a number of
modules that were not reachable at all. Some of those have been removed.
- This PR removes a usage of `windows_modules!` (since `cargo shear` and
`rustfmt` couldn't see through it) in favor of simple `#[cfg(target_os =
"windows")]` macros. As a consequence, many of these files exhibit churn
in this PR, since they weren't being formatted by `rustfmt` at all on
main.
- Again, to make the code more analyzable, this PR also removes some
usages of `#[path = "cwd_junction.rs"]` in favor of a more standard
module structure. The bin sidecar structure is still retained, but,
e.g., `windows-sandbox-rs/src/bin/command_runner.rs‎` was moved to
`windows-sandbox-rs/src/bin/command_runner/main.rs`, and so on.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/windows-sandbox-rs/src/hide_users.rs

@@ -1,5 +1,3 @@
-#![cfg(target_os = "windows")]
-
 use crate::logging::log_note;
 use crate::winutil::format_last_error;
 use crate::winutil::to_wide;
@@ -8,19 +6,19 @@ use std::ffi::OsStr;
 use std::path::Path;
 use std::path::PathBuf;
 use windows_sys::Win32::Foundation::GetLastError;
-use windows_sys::Win32::Storage::FileSystem::GetFileAttributesW;
-use windows_sys::Win32::Storage::FileSystem::SetFileAttributesW;
 use windows_sys::Win32::Storage::FileSystem::FILE_ATTRIBUTE_HIDDEN;
 use windows_sys::Win32::Storage::FileSystem::FILE_ATTRIBUTE_SYSTEM;
+use windows_sys::Win32::Storage::FileSystem::GetFileAttributesW;
 use windows_sys::Win32::Storage::FileSystem::INVALID_FILE_ATTRIBUTES;
-use windows_sys::Win32::System::Registry::RegCloseKey;
-use windows_sys::Win32::System::Registry::RegCreateKeyExW;
-use windows_sys::Win32::System::Registry::RegSetValueExW;
+use windows_sys::Win32::Storage::FileSystem::SetFileAttributesW;
 use windows_sys::Win32::System::Registry::HKEY;
 use windows_sys::Win32::System::Registry::HKEY_LOCAL_MACHINE;
 use windows_sys::Win32::System::Registry::KEY_WRITE;
 use windows_sys::Win32::System::Registry::REG_DWORD;
 use windows_sys::Win32::System::Registry::REG_OPTION_NON_VOLATILE;
+use windows_sys::Win32::System::Registry::RegCloseKey;
+use windows_sys::Win32::System::Registry::RegCreateKeyExW;
+use windows_sys::Win32::System::Registry::RegSetValueExW;
 
 const USERLIST_KEY_PATH: &str =
     r"SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList";