fix: resolve test_username.py import for plugins/ directory

Load plugins.username via importlib.util.spec_from_file_location
since plugins/ is not a Python package on sys.path.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_username.py

@@ -1,6 +1,18 @@
 """Tests for the username enumeration plugin."""
 
-from plugins.username import (
+import importlib.util
+import sys
+from pathlib import Path
+
+# plugins/ is not a Python package -- load the module from file path
+_spec = importlib.util.spec_from_file_location(
+    "plugins.username", Path(__file__).resolve().parent.parent / "plugins" / "username.py",
+)
+_mod = importlib.util.module_from_spec(_spec)
+sys.modules[_spec.name] = _mod
+_spec.loader.exec_module(_mod)
+
+from plugins.username import (  # noqa: E402
     _BY_CATEGORY,
     _BY_NAME,
     _SERVICES,