refactor: organize tests into unit/ and integration/ directories

Move unit tests from tests/ to tests/unit/ for clearer separation
from integration tests. Update pyproject.toml testpaths and Makefile
test target to reflect the new structure.

pyproject.toml

@@ -25,4 +25,4 @@ build-backend = "hatchling.build"
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
-testpaths = ["tests"]
+testpaths = ["tests/unit", "tests/integration"]