fix: issue 1138 windows encoding (#1139)

* fix(windows): use utf-8 for text file operations

* fix(windows): normalize sandbox path masking

* fix(windows): preserve utf-8 handling after backend split

backend/tests/test_local_sandbox_encoding.py

@@ -0,0 +1,33 @@
+import builtins
+
+import deerflow.sandbox.local.local_sandbox as local_sandbox
+from deerflow.sandbox.local.local_sandbox import LocalSandbox
+
+
+def _open(base, file, mode="r", *args, **kwargs):
+    if "b" in mode:
+        return base(file, mode, *args, **kwargs)
+    return base(file, mode, *args, encoding=kwargs.pop("encoding", "gbk"), **kwargs)
+
+
+def test_read_file_uses_utf8_on_windows_locale(tmp_path, monkeypatch):
+    path = tmp_path / "utf8.txt"
+    text = "\u201cutf8\u201d"
+    path.write_text(text, encoding="utf-8")
+    base = builtins.open
+
+    monkeypatch.setattr(local_sandbox, "open", lambda file, mode="r", *args, **kwargs: _open(base, file, mode, *args, **kwargs), raising=False)
+
+    assert LocalSandbox("t").read_file(str(path)) == text
+
+
+def test_write_file_uses_utf8_on_windows_locale(tmp_path, monkeypatch):
+    path = tmp_path / "utf8.txt"
+    text = "emoji \U0001F600"
+    base = builtins.open
+
+    monkeypatch.setattr(local_sandbox, "open", lambda file, mode="r", *args, **kwargs: _open(base, file, mode, *args, **kwargs), raising=False)
+
+    LocalSandbox("t").write_file(str(path), text)
+
+    assert path.read_text(encoding="utf-8") == text