Alerting: Persist AlertInstance ResolvedAt & LastSentAt (#89135)

* Alerting: Persist AlertInstance ResolvedAt & LastSentAt

* Fix test

* Modify existing tests

* Fix merge conflicts from nullable LastSentAt & ResolvedAt

pkg/services/ngalert/models/testing.go

@@ -17,6 +17,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	alertingModels "github.com/grafana/alerting/models"
+
 	"github.com/grafana/grafana/pkg/expr"
 	"github.com/grafana/grafana/pkg/services/datasources"
 	"github.com/grafana/grafana/pkg/services/folder"
@@ -840,6 +841,11 @@ func AlertInstanceGen(mutators ...AlertInstanceMutator) *AlertInstance {
 		CurrentStateSince: currentStateSince,
 		CurrentStateEnd:   currentStateSince.Add(time.Duration(rand.Intn(100) + 200)),
 		LastEvalTime:      time.Now().Add(-time.Duration(rand.Intn(100) + 50)),
+		LastSentAt:        util.Pointer(time.Now().Add(-time.Duration(rand.Intn(100) + 50))),
+	}
+
+	if instance.CurrentState == InstanceStateNormal && rand.Intn(2) == 1 {
+		instance.ResolvedAt = util.Pointer(time.Now().Add(-time.Duration(rand.Intn(100) + 50)))
 	}
 
 	for _, mutator := range mutators {