Preserve Windows environment variables for remote MCP servers (#50129)

## Why

When Codex runs on Unix and launches a stdio MCP server on a Windows executor, explicit remote environment variables activate an allowlist based on Unix defaults. This can filter out Windows runtime and temporary-directory variables.

## What changed

Include `SYSTEMROOT`, `TEMP`, and `TMP` in the remote environment allowlist whenever explicit remote variables are requested.

## Testing

Extend the environment-filtering test to verify that `SystemRoot`, `TEMP`, and `TMP` survive alongside requested variables while unrequested secrets remain excluded.

GitOrigin-RevId: 059c9e23833be66aaf5b65ba5f95013f94f0f792

codex-rs/rmcp-client/src/stdio_server_launcher.rs

@@ -687,8 +687,12 @@ impl ExecutorStdioServerLauncher {
             // environment, not copied from Codex. Start from `All` only so the
             // named remote variable is available to the filter below; the
             // effective child env is still limited by `include_only`.
+            // The orchestrator may be Unix while the executor is Windows.
+            // Preserve Windows runtime initialization and temporary-directory
+            // inputs even when explicit remote vars activate this allowlist.
             crate::utils::DEFAULT_ENV_VARS
                 .iter()
+                .chain(["SYSTEMROOT", "TEMP", "TMP"].iter())
                 .map(|name| (*name).to_string())
                 .chain(remote_env_vars.iter().cloned())
                 .collect()
@@ -763,6 +767,15 @@ mod tests {
         let env = shell_environment::create_env_from_vars(
             [
                 ("PATH".to_string(), "/remote/bin".to_string()),
+                ("SystemRoot".to_string(), r"C:\Windows".to_string()),
+                (
+                    "TEMP".to_string(),
+                    r"C:\Users\test\AppData\Local\Temp".to_string(),
+                ),
+                (
+                    "TMP".to_string(),
+                    r"C:\Users\test\AppData\Local\Temp".to_string(),
+                ),
                 ("REMOTE_TOKEN".to_string(), "remote-secret".to_string()),
                 (
                     "UNREQUESTED_SECRET".to_string(),
@@ -774,6 +787,16 @@ mod tests {
         );
 
         assert_eq!(env.get("PATH").map(String::as_str), Some("/remote/bin"));
+        assert_eq!(
+            env.get("SystemRoot").map(String::as_str),
+            Some(r"C:\Windows")
+        );
+        for name in ["TEMP", "TMP"] {
+            assert_eq!(
+                env.get(name).map(String::as_str),
+                Some(r"C:\Users\test\AppData\Local\Temp")
+            );
+        }
         assert_eq!(
             env.get("REMOTE_TOKEN").map(String::as_str),
             Some("remote-secret")