Alerting: Fix "Not Implemented" responses (#57710)

* fix swagger spec, return 404 instead of 501 when an endpoint does not exist

* update number of paths in authorization_test.go

pkg/services/ngalert/api/tooling/api.json

@@ -3301,6 +3301,7 @@
    "type": "object"
   },
   "alertGroups": {
+   "description": "AlertGroups alert groups",
    "items": {
     "$ref": "#/definitions/alertGroup"
    },
@@ -3405,6 +3406,7 @@
    "type": "object"
   },
   "gettableAlert": {
+   "description": "GettableAlert gettable alert",
    "properties": {
     "annotations": {
      "$ref": "#/definitions/labelSet"
@@ -3460,6 +3462,7 @@
    "type": "object"
   },
   "gettableAlerts": {
+   "description": "GettableAlerts gettable alerts",
    "items": {
     "$ref": "#/definitions/gettableAlert"
    },
@@ -3664,7 +3667,6 @@
    "type": "array"
   },
   "postableSilence": {
-   "description": "PostableSilence postable silence",
    "properties": {
     "comment": {
      "description": "comment",
@@ -3702,6 +3704,7 @@
    "type": "object"
   },
   "receiver": {
+   "description": "Receiver receiver",
    "properties": {
     "active": {
      "description": "active",