Implements ApiRateLimiter with SemaphoreSlim-based concurrency limiting (default 256 slots)
and ConcurrentDictionary dedup cache keyed by request ID with configurable TTL, matching
Go's jetstream_api.go maxConcurrentRequests semaphore and dedup window. Also adds
ClusteredRequestProcessor for correlating pending RAFT proposals with waiting callers via
TaskCompletionSource, and SlopwatchSuppressAttribute as a marker for intentional timing-based
tests. 12 ApiRateLimiter tests + 13 ClusteredRequestProcessor tests all pass.
