test: add pytest coverage for core modules

tests/test_plugins.py

@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from afs.plugins import discover_plugins
+from afs.schema import AFSConfig, PluginsConfig
+
+
+def test_discover_plugins_in_custom_dir(tmp_path: Path) -> None:
+    plugin_dir = tmp_path / "plugins"
+    plugin_dir.mkdir()
+    package_dir = plugin_dir / "afs_plugin_demo"
+    package_dir.mkdir()
+    (package_dir / "__init__.py").write_text("", encoding="utf-8")
+
+    plugins = PluginsConfig(plugin_dirs=[plugin_dir], auto_discover=True)
+    config = AFSConfig(plugins=plugins)
+
+    names = discover_plugins(config)
+    assert "afs_plugin_demo" in names