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_artifacts_router.py

@@ -0,0 +1,27 @@
+import asyncio
+from pathlib import Path
+
+from starlette.requests import Request
+
+import app.gateway.routers.artifacts as artifacts_router
+
+
+def test_get_artifact_reads_utf8_text_file_on_windows_locale(tmp_path, monkeypatch) -> None:
+    artifact_path = tmp_path / "note.txt"
+    text = "Curly quotes: \u201cutf8\u201d"
+    artifact_path.write_text(text, encoding="utf-8")
+
+    original_read_text = Path.read_text
+
+    def read_text_with_gbk_default(self, *args, **kwargs):
+        kwargs.setdefault("encoding", "gbk")
+        return original_read_text(self, *args, **kwargs)
+
+    monkeypatch.setattr(Path, "read_text", read_text_with_gbk_default)
+    monkeypatch.setattr(artifacts_router, "resolve_thread_virtual_path", lambda _thread_id, _path: artifact_path)
+
+    request = Request({"type": "http", "method": "GET", "path": "/", "headers": [], "query_string": b""})
+    response = asyncio.run(artifacts_router.get_artifact("thread-1", "mnt/user-data/outputs/note.txt", request))
+
+    assert bytes(response.body).decode("utf-8") == text
+    assert response.media_type == "text/plain"