Context

PR #25660 introduces single-gateway multi-agent routing as an MVP and explicitly leaves per-agent token budgeting / priority scheduling for follow-up.

Work

• Add a per-agent token bucket, priority queue, or equivalent scheduler so one busy agent/profile cannot starve others.
• Define default fairness behavior for the main profile and routed profiles.
• Add regression tests for concurrent routed messages across multiple agents.

Source

Follow-up from #25660.

Referenced by #25660.