test(custom): cover empty choices response handling

tests/test_custom_provider.py

@@ -0,0 +1,13 @@
+from types import SimpleNamespace
+
+from nanobot.providers.custom_provider import CustomProvider
+
+
+def test_custom_provider_parse_handles_empty_choices() -> None:
+    provider = CustomProvider()
+    response = SimpleNamespace(choices=[])
+
+    result = provider._parse(response)
+
+    assert result.finish_reason == "error"
+    assert "empty choices" in result.content