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/models/alert_rule.go

@@ -388,7 +388,7 @@ func PatchPartialAlertRule(existingRule *AlertRule, ruleToPatch *AlertRule) {
 	if ruleToPatch.NoDataState == "" {
 		ruleToPatch.NoDataState = existingRule.NoDataState
 	}
-	if ruleToPatch.For == 0 {
+	if ruleToPatch.For == -1 {
 		ruleToPatch.For = existingRule.For
 	}
 }

pkg/services/ngalert/models/alert_rule_test.go

@@ -113,9 +113,9 @@ func TestPatchPartialAlertRule(t *testing.T) {
 				},
 			},
 			{
-				name: "For is 0",
+				name: "For is -1",
 				mutator: func(r *AlertRule) {
-					r.For = 0
+					r.For = -1
 				},
 			},
 		}