test: cover phase latency pairing

internal/storage/mysql/operational_metrics_test.go

@@ -0,0 +1,34 @@
+package mysql
+
+import (
+	"testing"
+	"time"
+)
+
+func TestPairPhaseLatenciesSortsEventsAndUsesFirstValidTarget(t *testing.T) {
+	base := time.Date(2026, 9, 19, 0, 0, 0, 0, time.UTC)
+	got := pairPhaseLatencies(map[string][]phaseEvent{
+		"task-a": {
+			{kind: "task", id: "terminal", typeName: "task.succeeded", at: base.Add(4 * time.Second)},
+			{kind: "task", id: "start", typeName: "task.started", at: base.Add(2 * time.Second)},
+			{kind: "task", id: "accepted", typeName: "task.accepted", at: base},
+			{kind: "task", id: "later-terminal", typeName: "task.failed", at: base.Add(5 * time.Second)},
+		},
+	}, "task.accepted", "task.succeeded", "task.failed")
+	if got.average != 4 || got.maximum != 4 {
+		t.Fatalf("phase latency=%+v, want first valid terminal at four seconds", got)
+	}
+}
+
+func TestPairPhaseLatenciesRejectsTargetBeforeStart(t *testing.T) {
+	base := time.Date(2026, 9, 19, 0, 0, 0, 0, time.UTC)
+	got := pairPhaseLatencies(map[string][]phaseEvent{
+		"workflow-a": {
+			{kind: "workflow", id: "stop", typeName: "workflow.stopped", at: base},
+			{kind: "workflow", id: "start", typeName: "workflow.started", at: base.Add(time.Second)},
+		},
+	}, "workflow.started", "workflow.stopped")
+	if got.average != 0 || got.maximum != 0 {
+		t.Fatalf("invalid phase pair produced latency=%+v", got)
+	}
+}