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_graph.py

@@ -2,13 +2,15 @@
 
 from __future__ import annotations
 
+from collections.abc import Callable
 from pathlib import Path
 
 import pytest
 
 from bulkgen.config import ProjectConfig
 from bulkgen.graph import build_graph, get_build_order, get_subgraph_for_target
-from tests.conftest import WriteConfig
+
+WriteConfig = Callable[[dict[str, object]], ProjectConfig]
 
 
 class TestBuildGraph: