Pin text I/O to UTF-8 and fail CI on locale-dependent reads/writes (#3296)

Co-authored-by: ShuQingDollarVoyager <57471784+ShuQingDollarVoyager@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/cli/test_utils.py

@@ -28,7 +28,7 @@ def _set_mcp_version(monkeypatch: pytest.MonkeyPatch, version: str) -> None:
 def test_parse_file_path_accepts_valid_specs(tmp_path: Path, spec: str, expected_obj: str | None):
     """Should accept valid file specs."""
     file = tmp_path / spec.split(":")[0]
-    file.write_text("x = 1")
+    file.write_text("x = 1", encoding="utf-8")
     path, obj = _parse_file_path(f"{file}:{expected_obj}" if ":" in spec else str(file))
     assert path == file.resolve()
     assert obj == expected_obj