test [nfc]: Pull out an example UNAUTHORIZED API exception, and add doc

Prompted by seeing we'll need more copies of this soon:
  #1183 (comment)

Author: gnprice

test/example_data.dart

@@ -1,6 +1,7 @@
 import 'dart:convert';
 import 'dart:math';
 
+import 'package:zulip/api/exception.dart';
 import 'package:zulip/api/model/events.dart';
 import 'package:zulip/api/model/initial_snapshot.dart';
 import 'package:zulip/api/model/model.dart';
@@ -18,10 +19,28 @@ void _checkPositive(int? value, String description) {
   assert(value == null || value > 0, '$description should be positive');
 }
 
+////////////////////////////////////////////////////////////////
+// Error objects.
+//
+
 Object nullCheckError() {
   try { null!; } catch (e) { return e; } // ignore: null_check_always_fails
 }
 
+/// The error the server gives when the client's credentials
+/// (API key together with email and realm URL) are no longer valid.
+///
+/// This isn't really documented, but comes from experiment and from
+/// reading the server implementation.  See:
+///   https://github.com/zulip/zulip-flutter/pull/1183#discussion_r1945865983
+///   https://chat.zulip.org/#narrow/channel/378-api-design/topic/general.20handling.20HTTP.20status.20code.20401/near/2090024
+ZulipApiException apiExceptionUnauthorized({String routeName = 'someRoute'}) {
+  return ZulipApiException(
+    routeName: routeName,
+    httpStatus: 401, code: 'UNAUTHORIZED',
+    data: {}, message: 'Invalid API key');
+}
+
 ////////////////////////////////////////////////////////////////
 // Realm-wide (or server-wide) metadata.
 //

test/model/actions_test.dart

@@ -102,13 +102,7 @@ void main() {
       check(testBinding.globalStore).accountIds.single.equals(eg.selfAccount.id);
       const unregisterDelay = Duration(seconds: 5);
       assert(unregisterDelay > TestGlobalStore.removeAccountDuration);
-      final exception = ZulipApiException(
-        httpStatus: 401,
-        code: 'UNAUTHORIZED',
-        data: {},
-        routeName: 'removeEtcEtcToken',
-        message: 'Invalid API key',
-      );
+      final exception = eg.apiExceptionUnauthorized(routeName: 'removeEtcEtcToken');
       final newConnection = separateConnection()
         ..prepare(delay: unregisterDelay, exception: exception);
 
@@ -170,14 +164,9 @@ void main() {
     test('connection closed if request errors', () => awaitFakeAsync((async) async {
       await prepare(ackedPushToken: '123');
 
+      final exception = eg.apiExceptionUnauthorized(routeName: 'removeEtcEtcToken');
       final newConnection = separateConnection()
-        ..prepare(exception: ZulipApiException(
-            httpStatus: 401,
-            code: 'UNAUTHORIZED',
-            data: {},
-            routeName: 'removeEtcEtcToken',
-            message: 'Invalid API key',
-          ));
+        ..prepare(exception: exception);
       final future = unregisterToken(testBinding.globalStore, eg.selfAccount.id);
       async.elapse(Duration.zero);
       await future;