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/env.rs

@@ -1,10 +1,13 @@
-use anyhow::{anyhow, Result};
+use anyhow::Result;
+use anyhow::anyhow;
 use dirs_next::home_dir;
 use std::collections::HashMap;
 use std::env;
-use std::fs::{self, File};
+use std::fs::File;
+use std::fs::{self};
 use std::io::Write;
-use std::path::{Path, PathBuf};
+use std::path::Path;
+use std::path::PathBuf;
 
 pub fn normalize_null_device_env(env_map: &mut HashMap<String, String>) {
     let keys: Vec<String> = env_map.keys().cloned().collect();