regression: add chat.sendMessage rate limiter - #41966

No issues found. Spectre coverage incomplete: selected 5, scanned 5, skipped 0, oversized 0, capped 1, truncated 0, failed 0, invalid 0, rejected 0, repairs 0, repaired responses 0, repaired findings 0, cancelled 0, rate-limited 0, concurrency-limited 0, circuit-open 0. Reason: file-cap-exceeded.