Fix reorg test flake (#1542)

For some weird reasons, the health check call in nodes doesn't show
finality violated error consistently across all the nodes. Due to which
this test
TestSmokeCCIPReorgAboveFinalityAtDestination/Above_finality_reorg_in_destination_chain
flakes often in CI.

Adjusting the expectation to have at least half of the nodes detects
violation instead of every node.
CI Failure examples:
https://github.com/smartcontractkit/chainlink/actions/runs/11894867899/job/33143516553

https://github.com/smartcontractkit/chainlink/actions/runs/11894232425/job/33141414438?pr=15282

integration-tests/ccip-tests/smoke/ccip_test.go

@@ -2,6 +2,7 @@ package smoke
 
 import (
 	"fmt"
+	"math"
 	"math/big"
 	"strings"
 	"testing"
@@ -892,7 +893,7 @@ func TestSmokeCCIPReorgBelowFinality(t *testing.T) {
 
 // Test creates above finality reorg at destination and
 // expects ccip transactions in-flight and the one initiated after reorg
-// doesn't go through and verifies every node is able to detect reorg.
+// doesn't go through and verifies f+1 nodes are able to detect reorg.
 // Note: LogPollInterval interval is set as 1s to detect the reorg immediately
 func TestSmokeCCIPReorgAboveFinalityAtDestination(t *testing.T) {
 	t.Parallel()
@@ -903,7 +904,7 @@ func TestSmokeCCIPReorgAboveFinalityAtDestination(t *testing.T) {
 
 // Test creates above finality reorg at destination and
 // expects ccip transactions in-flight doesn't go through, the transaction initiated after reorg
-// shouldn't even get initiated and verifies every node is able to detect reorg.
+// shouldn't even get initiated and verifies f+1 nodes are able to detect reorg.
 // Note: LogPollInterval interval is set as 1s to detect the reorg immediately
 func TestSmokeCCIPReorgAboveFinalityAtSource(t *testing.T) {
 	t.Parallel()
@@ -1059,11 +1060,13 @@ func performAboveFinalityReorgAndValidate(t *testing.T, network string) {
 		logPollerName = fmt.Sprintf("EVM.%d.LogPoller", lane.SourceChain.GetChainID())
 		rs.RunReorg(rs.SrcClient, int(rs.Cfg.SrcFinalityDepth)+rs.Cfg.FinalityDelta, network, 2*time.Second)
 	}
-	clNodes := setUpOutput.Env.CLNodes
-	// assert every node is detecting the reorg (LogPollInterval is set as 1s for faster detection)
+	// DON is 3F+1, finding f+1 from the given number of nodes in the environment
+	fPlus1Nodes := int(math.Ceil(float64(len(setUpOutput.Env.CLNodes)-1)/3)) + 1
+	// assert at least f+1 nodes is detecting the reorg (LogPollInterval is set as 1s for faster detection)
+	// additional context: Commit requires 2f+1 observations, so f+1 nodes need to detect it in order to force the entire DON to stop processing messages.
 	nodesDetectedViolation := make(map[string]bool)
 	assert.Eventually(t, func() bool {
-		for _, node := range clNodes {
+		for _, node := range setUpOutput.Env.CLNodes {
 			if _, ok := nodesDetectedViolation[node.ChainlinkClient.URL()]; ok {
 				continue
 			}
@@ -1076,8 +1079,8 @@ func performAboveFinalityReorgAndValidate(t *testing.T, network string) {
 				}
 			}
 		}
-		return len(nodesDetectedViolation) == len(clNodes)
-	}, 5*time.Minute, 5*time.Second, "Reorg above finality depth is not detected by every node")
+		return len(nodesDetectedViolation) >= fPlus1Nodes
+	}, 3*time.Minute, 20*time.Second, "Reorg above finality depth is not detected by f+1 nodes")
 	log.Debug().Interface("Nodes", nodesDetectedViolation).Msg("Violation detection details")
 	// send another request and verify it fails
 	err = lane.SendRequests(1, gasLimit)