V6.1 runtime settings ->.env.production fix

tests/ui/test_runtime_settings_store.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import errno
 from pathlib import Path
 
 import pytest
@@ -132,3 +133,18 @@ def test_save_runtime_settings_surfaces_unwritable_target(tmp_path: Path, monkey
     with pytest.raises(AppError) as exc:
         save_runtime_settings(settings=settings, updates={"port": "9001"}, env_file_path=env_path)
     assert exc.value.message == "Runtime settings file is not writable."
+
+
+def test_save_runtime_settings_falls_back_when_atomic_replace_is_unavailable(tmp_path: Path, monkeypatch):
+    settings = _settings_for_runtime_editing(tmp_path)
+    env_path = tmp_path / ".env.production"
+    env_path.write_text("OPENROUTER_API_KEY=test-key\nPORT=8000\n", encoding="utf-8")
+
+    def _replace_cross_device(_self: Path, _target: Path) -> Path:
+        raise OSError(errno.EXDEV, "Invalid cross-device link")
+
+    monkeypatch.setattr(Path, "replace", _replace_cross_device)
+    snapshot = save_runtime_settings(settings=settings, updates={"port": "9001"}, env_file_path=env_path)
+
+    assert snapshot.env_file_path == env_path
+    assert "PORT=9001" in env_path.read_text(encoding="utf-8")