fix: inline WriteConfig type alias in tests to fix nix flake check

The tests.conftest import could not be resolved in the nix sandbox
because tests is not a proper package. Define the WriteConfig type
alias directly in test_builder.py and test_graph.py instead.

tests/test_builder.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Callable
 from pathlib import Path
 from typing import override
 from unittest.mock import patch
@@ -18,7 +19,8 @@ from bulkgen.builder import (
 from bulkgen.config import ProjectConfig, TargetConfig, TargetType
 from bulkgen.providers import Provider
 from bulkgen.state import load_state
-from tests.conftest import WriteConfig
+
+WriteConfig = Callable[[dict[str, object]], ProjectConfig]
 
 
 class FakeProvider(Provider):