Handle missing virtualenv in shell command

[bubaizhanshen]

Summary

• replace the internal assertion when poetry shell is run without a virtualenv with a user-facing error
• avoid printing the misleading Spawning shell within ... line before the environment type is validated
• add regression coverage for non-virtualenv environments

Fixes #30.

Tests

• .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

Note: full mypy currently reports an existing tests/conftest.py override mismatch against Poetry 2.4.1, outside this change.