fix: avoid zero-rate queue reader throttling (#9619)

## Summary
- When `persistenceMaxQPS` is set to `0` (meaning "unlimited"), the
queue reader rate limiter receives a zero rate, causing all queue
readers to fail with "burst size is smaller than required token count"
and never process tasks.
- This fixes the rate function to return a high default (100,000) when
persistence QPS is unlimited, and adds a missing `return` after the
error log to prevent falling through to an invalid lock/read path.

## Bug Details
`NewHostRateLimiterRateFn` computes `float64(persistenceMaxRPS()) *
persistenceMaxRPSRatio`. When `persistenceMaxRPS()` returns 0
(unlimited), this produces rate=0, which creates a rate limiter with
burst=0. The reader then cannot acquire even 1 token, logging the error
but continuing into `r.Lock()` without tasks — effectively a tight error
loop.

## Testing
- Verified with benchmarks that queue readers function correctly with
`persistenceMaxQPS: 0` in dynamic config.
- Existing unit tests pass.

service/history/queue_factory_base.go

@@ -226,9 +226,17 @@ func NewHostRateLimiterRateFn(
 			return float64(maxPollHostRps)
 		}
 
-		// ensure queue loading won't consume all persistence tokens
-		// especially upon host restart when we need to perform a load
-		// for all shards
-		return float64(persistenceMaxRPS()) * persistenceMaxRPSRatio
+		if pMaxRPS := persistenceMaxRPS(); pMaxRPS > 0 {
+			// ensure queue loading won't consume all persistence tokens
+			// especially upon host restart when we need to perform a load
+			// for all shards
+			return float64(pMaxRPS) * persistenceMaxRPSRatio
+		}
+
+		// persistenceMaxQPS=0 means "unlimited" — use a high default to avoid
+		// producing a zero rate which would block all queue readers.
+		// NOTE: Do not use math.MaxFloat64 here; int(math.MaxFloat64) overflows
+		// to math.MinInt64, which breaks burst calculation in the rate limiter.
+		return 100_000
 	}
 }

service/history/queues/reader.go

@@ -431,6 +431,7 @@ func (r *ReaderImpl) loadAndSubmitTasks() {
 
 		// this should never happen
 		r.logger.Error("Queue reader rate limiter burst size is smaller than required token count")
+		return
 	}
 
 	r.Lock()