[codex] Migrate project tooling to uv

[ItayTheDar]

Summary

• Migrate package metadata from Poetry-specific tables to PEP 621 with Hatchling as the build backend.
• Replace Poetry usage in tests, docs, release, beta release, and publishing workflows with uv.
• Keep library-style behavior by ignoring uv.lock, while documenting uv development/build commands.

Impact

• Contributors can use uv sync, uv run, and uv build for local development.
• GitHub Actions now install uv, build artifacts with uv build, validate them with Twine, smoke-test wheel/sdist artifacts, and publish with uv publish.
• Runtime nest.__version__ is aligned with the published package version during releases.

Validation

• git diff --check
• uv sync --no-dev --group test
• uv run pytest tests (22 passed, 1 existing pytest collection warning)
• uv sync --no-dev --group build
• uv build --sdist --wheel
• uv run twine check dist/*
• UV_PUBLISH_USERNAME=__token__ UV_PUBLISH_PASSWORD=dummy-token uv publish --dry-run
• isolated wheel and sdist import smoke tests
• CLI smoke test from built wheel
• uv sync --no-dev --group docs
• uv run mkdocs build --clean
• actionlint on all workflows

Notes

• Real PyPI/TestPyPI upload still depends on the existing GitHub secrets being valid: RELEASE_GIT_TOKEN, PYPI_API_USER, PYPI_API_TOKEN, TEST_PYPI_API_USER, and TEST_PYPI_API_TOKEN.