test: add unicode preservation and round-trip tests for save_vault_yaml

Add tests verifying allow_unicode=True behavior:
- test_save_preserves_unicode: non-ASCII content (Cyrillic, German
  umlaut, Chinese) is written as UTF-8, not \uXXXX escapes
- test_save_unicode_round_trip: saved unicode data loads back correctly

Also add CHANGELOG entry under [Unreleased] > Fixed.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/unit/test_utils_vault.py

@@ -75,6 +75,25 @@ class TestSaveVaultYaml:
         content = f.read_text()
         assert "key: value" in content
 
+    def test_save_preserves_unicode(self, tmp_path: Path) -> None:
+        """allow_unicode=True writes UTF-8 instead of \\uXXXX escapes."""
+        f = tmp_path / "output.yml"
+        data = {"name": "Тест", "city": "München", "role": "管理员"}
+        save_vault_yaml(f, data)
+        content = f.read_text()
+        assert "Тест" in content
+        assert "München" in content
+        assert "管理员" in content
+        assert "\\u" not in content
+
+    def test_save_unicode_round_trip(self, tmp_path: Path) -> None:
+        """Saved unicode data loads back correctly."""
+        f = tmp_path / "output.yml"
+        data = {"name": "Тест", "city": "München"}
+        save_vault_yaml(f, data)
+        loaded = load_vault_yaml(f)
+        assert loaded == data
+
     def test_save_with_vault_pass_not_exists(self, tmp_path: Path) -> None:
         f = tmp_path / "output.yml"
         vp = tmp_path / "nonexistent"