Alerting: Hook up GMA silence APIs to new authentication handler (#86625)

This PR connects the new RBAC authentication service to existing alertmanager API silence endpoints.

pkg/tests/api/alerting/api_alertmanager_test.go

@@ -1996,13 +1996,18 @@ func TestIntegrationAlertmanagerCreateSilence(t *testing.T) {
 
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
-			silenceID, err := client.PostSilence(t, tc.silence)
+			silenceOkBody, status, body := client.PostSilence(t, tc.silence)
+			t.Log(body)
 			if tc.expErr != "" {
-				require.EqualError(t, err, tc.expErr)
-				require.Empty(t, silenceID)
+				assert.NotEqual(t, http.StatusAccepted, status)
+
+				var validationError errutil.PublicError
+				assert.NoError(t, json.Unmarshal([]byte(body), &validationError))
+				assert.Contains(t, validationError.Message, tc.expErr)
+				assert.Empty(t, silenceOkBody.SilenceID)
 			} else {
-				require.NoError(t, err)
-				require.NotEmpty(t, silenceID)
+				assert.Equal(t, http.StatusAccepted, status)
+				assert.NotEmpty(t, silenceOkBody.SilenceID)
 			}
 		})
 	}