Fix Windows doctor npm root probe (#22967)

## Why
On Windows npm-managed installs expose the working shim as `npm.cmd`.
`codex doctor` probed bare `npm`, which could incorrectly report that
npm global-root inspection was unavailable even when the install was
healthy.

Fixes #22964.

## What changed
- Use `npm.cmd` for the doctor npm-root probe on Windows.
- Keep the existing `npm` probe on non-Windows platforms.

codex-rs/cli/src/doctor.rs

@@ -105,6 +105,10 @@ const COLOR_ENV_VARS: &[&str] = &[
 const TERMINAL_DIMENSION_ENV_VARS: &[&str] = &["COLUMNS", "LINES"];
 const TERMINFO_ENV_VARS: &[&str] = &["TERMINFO", "TERMINFO_DIRS"];
 const LOCALE_ENV_VARS: &[&str] = &["LC_ALL", "LC_CTYPE", "LANG"];
+#[cfg(windows)]
+const NPM_COMMAND: &str = "npm.cmd";
+#[cfg(not(windows))]
+const NPM_COMMAND: &str = "npm";
 const REMOTE_TERMINAL_ENV_VARS: &[&str] = &[
     "SSH_TTY",
     "SSH_CONNECTION",
@@ -884,7 +888,7 @@ fn npm_global_root_check() -> NpmRootCheck {
         return NpmRootCheck::MissingPackageRoot;
     };
 
-    let output = match run_command("npm", ["root", "-g"]) {
+    let output = match run_command(NPM_COMMAND, ["root", "-g"]) {
         Ok(output) => output,
         Err(err) => return NpmRootCheck::NpmUnavailable(err),
     };