CI: gate PyPI releases on automated tests and code quality checks

[FernandoCelmer]

Context

This repo currently publishes to PyPI (python-publish-pypi.yml / python-publish-pypi-test.yml) with no automated verification step: no test suite runs, no linting/type-checking gate, and no check that LAST_VERSION, pyproject.toml, and mkdocs_dracula_theme/__init__.py actually agree on the version being released. A release can currently ship even if those three drift out of sync.

Goal

Add a CI safety net so releases only go out once tests and code-quality checks pass, and once the version is verified consistent across all three sources of truth.

Concepts / building blocks

• verify-version action — new composite action that compares LAST_VERSION against pyproject.toml's version and mkdocs_dracula_theme/__init__.py's __version__, failing the run on any mismatch.
• test.yml workflow — runs pytest (with coverage) across a Python version matrix (3.9–3.13), reusable via workflow_call.
• code-quality.yml workflow — runs ruff check, ruff format --check, and mypy across the same matrix, reusable via workflow_call.
• .code_quality/ — new ruff.toml and mypy.ini config, since none exist yet.
• tests/ — new directory with a minimal smoke-test suite for the theme package, since none exists yet.
• Poetry dependency groups — add dev (pytest, pytest-cov) and code-quality (ruff, mypy) groups to pyproject.toml.
• Publish workflow gating — python-publish-pypi.yml and python-publish-pypi-test.yml call test.yml + code-quality.yml as required jobs before the deploy job runs, and call verify-version before building the package.
• setup-poetry action fix — cache key currently ignores the Python version, so cross-version runs can collide on the same cache entry; key needs ${{ inputs.python-version }} added.

Scope

• Add verify-version composite action
• Fix setup-poetry cache key (include Python version) + add poetry lock fallback
• Add test.yml workflow + minimal tests/ suite
• Add code-quality.yml workflow + .code_quality/ruff.toml + .code_quality/mypy.ini
• Add dev and code-quality poetry dependency groups to pyproject.toml
• Gate python-publish-pypi.yml on tests + code quality + version verification
• Gate python-publish-pypi-test.yml on tests + code quality + version verification