Improve Beam CLI upgrade guidance

[luke-lombardi]

What changed

• show the resolved Python interpreter behind the running beam command
• recommend upgrading with that interpreter instead of a potentially unrelated bare pip
• add a post-upgrade beam --version verification step
• declare packaging as a direct runtime dependency
• add regression coverage for the upgrade notice

Why

A user can run beam through a pyenv shim while a bare pip points at another Python environment. In that case, pip install --upgrade beam-client may report a new package version without updating the Beam CLI that is actually running. A clean wheel installation also exposed that the version-check module imported packaging without declaring it directly, which could crash the CLI before the guidance appeared.

Impact

The update-required message is now actionable for pyenv, virtualenv, and other multi-Python setups: it identifies the real environment, provides a copyable command that upgrades that exact installation, and tells the user how to verify the result.

Validation

• .venv/bin/pytest -q
• .venv/bin/ruff check src tests
• .venv/bin/python -m build --wheel
• isolated clean-wheel test through a shimmed beam executable against a mock minimum-version endpoint