Fix dev dependency install flow in Poetry setup

[NayerBasim]

Summary

Fixes #1097 by moving development dependencies to Poetry dependency groups and updating install commands accordingly.

Why this update is needed

Poetry recommends dependency groups for development-process dependencies and reserves extras for runtime optional features:

• Poetry states non-main groups should contain development-process dependencies, while runtime add-ons should use extras: https://python-poetry.org/docs/main/managing-dependencies/#dependency-groups
• [tool.poetry.extras] is deprecated (prefer [project.optional-dependencies] for extras): https://python-poetry.org/docs/main/pyproject/#extras

What changed

• Moved dev/docs tooling dependencies to [tool.poetry.group.dev.dependencies] in pyproject.toml
• Updated contributor setup command to poetry install --with dev in CONTRIBUTING.md
• Updated CI setup action to use --with dev in .github/actions/setup-python/action.yaml

Validation

• poetry install --with dev succeeds
• poetry run python -c "import pytest_cases" succeeds
• poetry run pytest -v tests --ignore tests/integration succeeds