Ports the rate-limit half of main's #8358 into the branch's reworked XAi
extension: a prompt_complete carrying stopReason rate_limit now fails the
hung prompt with the -32003 usage-limit error instead of settling it as a
normal end_turn, so the turn surfaces "usage limit reached" rather than
silently ending. The prompt-completion deferreds carry the error channel
end to end.

The exit_plan_mode approval gate from #8358 is NOT ported here: it needs
a v2 plan-flow design in AcpAdapterV2 (the v1 GrokAdapter it lived in is
gone) and is tracked separately.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>