AssertionError when spawning shell with "virtualenvs.create" set to false

[dunossauro]

When attempting to activate the shell in a project with the virtualenvs.create configuration set to false, an AssertionError is raised. While the error itself may not be inherently wrong (since Poetry is expecting a virtual environment), the error message should more clearly indicate that the virtualenvs.create setting has been disabled, preventing the creation of a virtual environment.

How to reproduce:

1. Create a new project:

   $ poetry new env-issue
   Created package env_issue in env-issue
   $ cd env-issue/

2. Disable virtual environment creation:

   poetry config virtualenvs.create false --local

3. Attempt to activate the Poetry shell:

   $ poetry shell
   Skipping virtualenv creation, as specified in config file.
   Spawning shell within /usr
   
     AssertionError

Error traceback:

  at ~/.local/share/pipx/venvs/poetry/lib/python3.13/site-packages/poetry_plugin_shell/command.py:43 in handle
       39│         self.line(f"Spawning shell within {self.env.path}")
       40│ 
       41│         # Be sure that we have the right type of environment.
       42│         env = self.env
    →  43│         assert env.is_venv()
       44│         env = cast("VirtualEnv", env)
       45│ 
       46│         # Setting this to avoid spawning unnecessary nested shells
       47│         os.environ["POETRY_ACTIVE"] = "1"

Expected behavior:

The shell may indeed not be spawned, and that is correct. However, instead of the AssertionError, the error message should clearly indicate that the virtualenvs.create setting is disabled, which prevents Poetry from creating a virtual environment. A more descriptive message, such as "Virtual environment creation is disabled by the configuration," would help clarify the reason for the error.