`from __future__ import annotations` turned the set_steps tool's
`context_variables: ContextVariables` param into an unresolved ForwardRef at
AG2 tool-schema-generation time, raising PydanticUserError on import and failing
the showcase-ag2 staging healthcheck since 2026-05-31. Removing it matches the
working sibling agents. Adds a regression test that statically asserts the
future-import stays absent (version-independent) plus a live import check.