Alerting: Fix rule API to accept 0 duration of field For (#50992)

* make 'for' pointer to distinguish between missing field and 0
* set 'for' to -1 if the value is missing but not allow negative in the request + path -1 with the value from original rule
* update store validation to not allow negative 'for'
* update usages to use pointer

pkg/services/ngalert/api/api_ruler_validation.go

@@ -110,8 +110,13 @@ func validateRuleNode(
 		ExecErrState:    errorState,
 	}
 
+	var err error
+	newAlertRule.For, err = validateForInterval(ruleNode)
+	if err != nil {
+		return nil, err
+	}
+
 	if ruleNode.ApiRuleNode != nil {
-		newAlertRule.For = time.Duration(ruleNode.ApiRuleNode.For)
 		newAlertRule.Annotations = ruleNode.ApiRuleNode.Annotations
 		newAlertRule.Labels = ruleNode.ApiRuleNode.Labels
 
@@ -131,10 +136,24 @@ func validateRuleNode(
 			newAlertRule.PanelID = &panelIDValue
 		}
 	}
-
 	return &newAlertRule, nil
 }
 
+// validateForInterval validates ApiRuleNode.For and converts it to time.Duration. If the field is not specified returns 0 if GrafanaManagedAlert.UID is empty and -1 if it is not.
+func validateForInterval(ruleNode *apimodels.PostableExtendedRuleNode) (time.Duration, error) {
+	if ruleNode.ApiRuleNode == nil || ruleNode.ApiRuleNode.For == nil {
+		if ruleNode.GrafanaManagedAlert.UID != "" {
+			return -1, nil // will be patched later with the real value of the current version of the rule
+		}
+		return 0, nil // if it's a new rule, use the 0 as the default
+	}
+	duration := time.Duration(*ruleNode.ApiRuleNode.For)
+	if duration < 0 {
+		return 0, fmt.Errorf("field `for` cannot be negative [%v]. 0 or any positive duration are allowed", *ruleNode.ApiRuleNode.For)
+	}
+	return duration, nil
+}
+
 // validateRuleGroup validates API model (definitions.PostableRuleGroupConfig) and converts it to a collection of models.AlertRule.
 // Returns a slice that contains all rules described by API model or error if either group specification or an alert definition is not valid.
 func validateRuleGroup(