Alerting: Add backend support for keep_firing_for (#100750)

What is this feature?

This PR introduces a new alert rule configuration option, keep_firing_for (Prometheus documentation).

keep_firing_for prevents alerts from resolving immediately after the alert condition returns to normal. Instead, they transition into a "Recovering" state and are not considered resolved by the Alertmanager. Once the recovery period ends (or after the next evaluation if it is bigger than keep_firing_for), the alert transitions to "Normal" if it doesn't start alerting again:

Before                                          

+----------+     +----------+                    
| Alerting |---->|  Normal  |                    
+----------+     +----------+                    

-----
After

+----------+      +------------+     +----------+
| Alerting |----->| Recovering |---->|  Normal  |
+----------+      +------------+     +----------+                                                 

Why do we need this feature?

This feature prevents flapping alerts by adding a recovery period. This helps avoid false resolutions caused by brief alert

pkg/services/ngalert/api/api_ruler.go

@@ -662,10 +662,12 @@ func toGettableExtendedRuleNode(r ngmodels.AlertRule, provenanceRecords map[stri
 		},
 	}
 	forDuration := model.Duration(r.For)
+	keepFiringForDuration := model.Duration(r.KeepFiringFor)
 	gettableExtendedRuleNode.ApiRuleNode = &apimodels.ApiRuleNode{
-		For:         &forDuration,
-		Annotations: r.Annotations,
-		Labels:      r.Labels,
+		For:           &forDuration,
+		KeepFiringFor: &keepFiringForDuration,
+		Annotations:   r.Annotations,
+		Labels:        r.Labels,
 	}
 	return gettableExtendedRuleNode
 }