fix: add isdir mock to filepath unit tests

Two tests that mock the storage backend were not setting
mock_backend.isdir.return_value = False. The new encode() logic
calls backend.isdir() before backend.size(), so without the mock
isdir() returned a truthy MagicMock, making is_dir=True and
skipping the size() call entirely (leaving size=None).

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: dimitri-yatsenko

tests/unit/test_codecs.py

@@ -548,6 +548,7 @@ def test_filepath_allows_user_paths(self):
             with patch("datajoint.hash_registry.get_store_backend") as mock_get_backend:
                 mock_backend = MagicMock()
                 mock_backend.exists.return_value = True
+                mock_backend.isdir.return_value = False
                 mock_backend.size.return_value = 1024
                 mock_get_backend.return_value = mock_backend
 
@@ -636,6 +637,7 @@ def test_filepath_enforces_filepath_prefix(self):
             with patch("datajoint.hash_registry.get_store_backend") as mock_get_backend:
                 mock_backend = MagicMock()
                 mock_backend.exists.return_value = True
+                mock_backend.isdir.return_value = False
                 mock_backend.size.return_value = 3072
                 mock_get_backend.return_value = mock_backend