fixes

tests/test_thumbnail.py

@@ -0,0 +1,22 @@
+from pathlib import Path
+from unittest.mock import patch
+
+from evanescere.services.thumbnail import select_character_overlay
+
+
+def test_select_character_overlay_returns_none_for_empty_directory(tmp_path: Path):
+    assert select_character_overlay(str(tmp_path)) is None
+
+
+def test_select_character_overlay_filters_png_files(tmp_path: Path):
+    first = tmp_path / "first.PNG"
+    second = tmp_path / "second.png"
+    first.write_bytes(b"")
+    second.write_bytes(b"")
+    (tmp_path / "notes.txt").write_text("ignore me")
+
+    with patch("evanescere.services.thumbnail.random.choice", return_value=second) as choice:
+        assert select_character_overlay(str(tmp_path)) == second
+
+    assert choice.call_args.args[0] == [first, second]
+