Port two refinements from the GLM variant onto the Claude base:
- prepareAgentStep: add a comment note that AI SDK v7 renames the per-step
`system` field to `instructions` (v6 ^6.0.134 still uses `system`), so it
gets updated correctly on the next SDK bump.
- ai-chat.service.spec: add an explicit off-by-one boundary test for
prepareAgentStep, expressed via MAX_AGENT_STEPS instead of a hardcoded 18/19
so it tracks the constant if the cap changes.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>