fix(cli): support legacy use_linux_sandbox_bwrap flag (#14473)

## Summary
- restore `use_linux_sandbox_bwrap` as a removed feature key so older
`--enable` callers parse again
- keep it as a no-op by leaving runtime behavior unchanged
- add regression coverage for the legacy `--enable` path

## Testing
- Not run (updated and pushed quickly)

codex-rs/cli/src/main.rs

@@ -1516,6 +1516,19 @@ mod tests {
         );
     }
 
+    #[test]
+    fn feature_toggles_accept_legacy_linux_sandbox_flag() {
+        let toggles = FeatureToggles {
+            enable: vec!["use_linux_sandbox_bwrap".to_string()],
+            disable: Vec::new(),
+        };
+        let overrides = toggles.to_overrides().expect("valid features");
+        assert_eq!(
+            overrides,
+            vec!["features.use_linux_sandbox_bwrap=true".to_string(),]
+        );
+    }
+
     #[test]
     fn feature_toggles_unknown_feature_errors() {
         let toggles = FeatureToggles {