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/exec-server/src/server/jsonrpc.rs

@@ -1,53 +0,0 @@
-use codex_app_server_protocol::JSONRPCError;
-use codex_app_server_protocol::JSONRPCErrorError;
-use codex_app_server_protocol::JSONRPCMessage;
-use codex_app_server_protocol::JSONRPCResponse;
-use codex_app_server_protocol::RequestId;
-use serde_json::Value;
-
-pub(crate) fn invalid_request(message: String) -> JSONRPCErrorError {
-    JSONRPCErrorError {
-        code: -32600,
-        data: None,
-        message,
-    }
-}
-
-pub(crate) fn invalid_params(message: String) -> JSONRPCErrorError {
-    JSONRPCErrorError {
-        code: -32602,
-        data: None,
-        message,
-    }
-}
-
-pub(crate) fn method_not_found(message: String) -> JSONRPCErrorError {
-    JSONRPCErrorError {
-        code: -32601,
-        data: None,
-        message,
-    }
-}
-
-pub(crate) fn response_message(
-    request_id: RequestId,
-    result: Result<Value, JSONRPCErrorError>,
-) -> JSONRPCMessage {
-    match result {
-        Ok(result) => JSONRPCMessage::Response(JSONRPCResponse {
-            id: request_id,
-            result,
-        }),
-        Err(error) => JSONRPCMessage::Error(JSONRPCError {
-            id: request_id,
-            error,
-        }),
-    }
-}
-
-pub(crate) fn invalid_request_message(reason: String) -> JSONRPCMessage {
-    JSONRPCMessage::Error(JSONRPCError {
-        id: RequestId::Integer(-1),
-        error: invalid_request(reason),
-    })
-}