test(ai-chat): pin step-limit boundary + note AI SDK v7 system->instructions

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>

apps/server/src/core/ai-chat/ai-chat.service.ts

@@ -38,6 +38,9 @@ const FINAL_STEP_INSTRUCTION =
 // `system` is the in-scope system prompt; we CONCATENATE so the original
 // persona/context is preserved — a bare `system` override would REPLACE the
 // whole system prompt for the step.
+//
+// NOTE: at AI SDK v7 the per-step `system` field is renamed to `instructions`.
+// On v6 (`^6.0.134`) `system` is the correct field — adjust when bumping.
 export function prepareAgentStep(
   stepNumber: number,
   system: string,