Handle missing virtualenv in shell command

[bubaizhanshen]

Fixes #30.

This replaces the internal assertion when poetry shell is run without a virtualenv with a user-facing error. It also avoids printing the misleading Spawning shell within ... line before the environment type is validated.

Checks:

• pytest tests/test_shell_command.py -q -n 0
• ruff check src/poetry_plugin_shell/command.py tests/test_shell_command.py
• ruff format --check src/poetry_plugin_shell/command.py tests/test_shell_command.py
• pre-commit run --all-files

[StantonMatt]

I checked this locally because it covers the virtualenvs.create=false path from #30.

On the PR branch, a scratch project with local virtualenvs.create false now returns exit code 1 with the new user-facing message instead of reaching the assertion, and it no longer prints the misleading Spawning shell within ... line first.

Local checks I ran on Python 3.12.13:

• .venv/bin/python -m pytest -q tests/test_shell_command.py::test_shell_without_virtualenv_shows_error
• .venv/bin/python -m pytest -q
• .venv/bin/python -m ruff check src/poetry_plugin_shell/command.py tests/test_shell_command.py
• .venv/bin/python -m mypy src/poetry_plugin_shell/command.py tests/test_shell_command.py

I do not see a blocking issue from this pass.