test: fix help docs test by adding _import_module to fake kernel

The test_run_help_prints_docstring_for_known_command test was failing
because the fake kernel lacked a _import_module method, causing the
module import to fail silently and fall through to the error message.

Add a working _fake_import_module that uses importlib to resolve
the module path, allowing run_help to read and print the docstring.

Author: Ziang Zhang

tests/test_help_docs.py

@@ -1,18 +1,24 @@
 from __future__ import annotations
 
+import importlib
 from types import SimpleNamespace
 
 from trushell.commands.core import run_help
 
 
 def test_run_help_prints_docstring_for_known_command(monkeypatch, capsys):
+    def _fake_import_module(path: str):
+        module = importlib.import_module(path.replace("/", ".").removesuffix(".py"))
+        return module
+
     fake_kernel = SimpleNamespace(
         registry={
             "settings": {
                 "path": "trushell/commands/settings.py",
                 "function": "run_settings",
             }
-        }
+        },
+        _import_module=_fake_import_module,
     )
 
     monkeypatch.setattr("trushell.core.trukernel.get_kernel", lambda: fake_kernel)