Add package building test to CI

[ayesha-usmani]

Summary

There is currently no CI check that verifies the Python package builds successfully (sdist and wheel). This means packaging regressions — such as missing files in MANIFEST.in, broken pyproject.toml metadata, or import errors in the built package — can go undetected until release time.

Motivation

Packaging issues have been hard to catch because nothing in CI actually runs python -m build or validates the resulting artifacts. The existing workflows cover unit tests, integration tests, pre-commit, and provider builds, but none exercise the package build itself.

Proposed Solution

Add a new CI workflow (or a job in an existing workflow) that:

1. Builds the sdist and wheel using python -m build
2. Installs the built wheel into a clean virtual environment
3. Validates the package with twine check dist/*
4. Optionally runs a basic smoke test (e.g., python -c "import llama_stack") against the installed wheel rather than the source tree

This should trigger on PRs that modify packaging-related files at minimum (pyproject.toml, MANIFEST.in, setup.cfg, etc.), and ideally on all PRs to catch transitive breakage.