test(conversations): Add makeTurn factory for turn-based tests

Replace the repeated inline turn objects in the turnsToMessages and
mergeEmptyTurns specs with a makeTurn factory so new turn fields (like
reasoning) only need a default in one place. Also cover reasoning flowing
through to the assistant message.

Refs TET-2453
Co-Authored-By: Claude <noreply@anthropic.com>

Author: Ognjen Bostjancic