rac2: fix incorrect assertion when send-queue becomes empty

The indexToSend in this case was being updated before the loop, but
the preciseSizeSum was incrementally deducted inside the loop. This
meant that if more than one item was being popped and the queue was
becoming empty, the code would incorrectly assert that preciseSizeSum
should be 0 after subtracting the tokens for the first entry.

Fixes #135289, #135344

Epic: none

Release note: None

pkg/kv/kvserver/kvflowcontrol/rac2/range_controller.go

@@ -2601,6 +2601,7 @@ func (rss *replicaSendStream) handleReadyEntriesRaftMuAndStreamLocked(
 		}
 		rss.mu.sendQueue.indexToSend = event.sendingEntries[n-1].id.index + 1
 		var sendTokensToDeduct [admissionpb.NumWorkClasses]kvflowcontrol.Tokens
+		var sendQPreciseSizeDelta kvflowcontrol.Tokens
 		for _, entry := range event.sendingEntries {
 			if !entry.usesFlowControl {
 				continue
@@ -2628,11 +2629,14 @@ func (rss *replicaSendStream) handleReadyEntriesRaftMuAndStreamLocked(
 			if inSendQueue && entry.id.index >= rss.mu.nextRaftIndexInitial {
 				// Was in send-queue and had eval tokens deducted for it.
 				rss.mu.sendQueue.originalEvalTokens[WorkClassFromRaftPriority(entry.pri)] -= tokens
-				rss.applySendQueuePreciseSizeDeltaRaftMuAndStreamLocked(ctx, -tokens)
+				sendQPreciseSizeDelta -= tokens
 			}
 			rss.raftMu.tracker.Track(ctx, entry.id, pri, tokens)
 			sendTokensToDeduct[WorkClassFromRaftPriority(pri)] += tokens
 		}
+		if sendQPreciseSizeDelta != 0 {
+			rss.applySendQueuePreciseSizeDeltaRaftMuAndStreamLocked(ctx, sendQPreciseSizeDelta)
+		}
 		flag := AdjNormal
 		if directive.preventSendQNoForceFlush {
 			flag = AdjPreventSendQueue