Preserve WINDIR in core Windows shell environments (#39809)

## What changed

- Add `WINDIR` to the Windows core environment variable allowlist.
- Extend the Windows core-inheritance test to verify that a case-variant
  `WinDir` entry is retained.

GitOrigin-RevId: 0305d8fa882ba2705a7ce7661098cedac1339c43

codex-rs/protocol/src/shell_environment.rs

@@ -172,6 +172,7 @@ pub const WINDOWS_CORE_ENV_VARS: &[&str] = &[
     "SHELL",
     "COMSPEC",
     "SYSTEMROOT",
+    "WINDIR",
     "SYSTEMDRIVE",
     // User context and profiles
     "USERNAME",
@@ -218,6 +219,7 @@ mod windows_tests {
         let vars = make_vars(&[
             ("Shell", "C:\\Program Files\\Git\\bin\\bash.exe"),
             ("SystemRoot", "C:\\Windows"),
+            ("WinDir", "C:\\Windows"),
             ("AppData", "C:\\Users\\codex\\AppData\\Roaming"),
             ("TmpDir", "C:\\Temp\\custom"),
             ("OPENAI_API_KEY", "secret"),
@@ -237,6 +239,7 @@ mod windows_tests {
                 "C:\\Program Files\\Git\\bin\\bash.exe".to_string(),
             ),
             ("SystemRoot".to_string(), "C:\\Windows".to_string()),
+            ("WinDir".to_string(), "C:\\Windows".to_string()),
             (
                 "AppData".to_string(),
                 "C:\\Users\\codex\\AppData\\Roaming".to_string(),