Preserve SystemRoot for Windows filesystem sandbox helpers (#56468)

## Why

MXC needs `SystemRoot` to create the sandboxed filesystem helper process.

## What changed

Allow `SystemRoot` through the Windows helper environment filter using case-insensitive matching.

## Testing

Extend the environment-filter test to cover mixed-case `SystemRoot` and the Windows sandbox write-policy test to verify that writes to an allowed directory succeed.

GitOrigin-RevId: 0979b0c83eff245ee380fae9d7cf05dcc739265f

codex-rs/exec-server/src/fs_sandbox.rs

@@ -373,10 +373,10 @@ fn helper_env_key_is_allowed(key: &str) -> bool {
         // CoreFoundation consults this before falling back to user lookup during helper startup.
         || (cfg!(target_os = "macos") && key == "__CF_USER_TEXT_ENCODING")
         || bazel_bwrap_env_key_is_allowed(key)
-        // MXC needs SystemDrive to resolve platform directories and LOCALAPPDATA
-        // to create the sandboxed helper process.
+        // MXC needs SystemDrive to resolve platform directories, and SystemRoot and
+        // LOCALAPPDATA to create the sandboxed helper process.
         || (cfg!(windows)
-            && ["PATH", "SystemDrive", "LOCALAPPDATA"]
+            && ["PATH", "SystemDrive", "SystemRoot", "LOCALAPPDATA"]
                 .iter()
                 .any(|allowed| key.eq_ignore_ascii_case(allowed)))
 }
@@ -716,6 +716,7 @@ mod tests {
                 ("Path", r"C:\Windows\System32"),
                 ("LocalAppData", r"C:\Users\test\AppData\Local"),
                 ("SystemDrive", "C:"),
+                ("systemROOT", r"C:\Windows"),
                 ("PATH_INJECTION", "bad"),
                 ("OPENAI_API_KEY", "secret"),
             ]
@@ -731,6 +732,7 @@ mod tests {
                     r"C:\Users\test\AppData\Local".to_string()
                 ),
                 ("SystemDrive".to_string(), "C:".to_string()),
+                ("systemROOT".to_string(), r"C:\Windows".to_string()),
             ])
         );
     }