test: Generate random, increasing message IDs for example data by default

This allows `eg.streamMessage` and `eg.dmMessage` to be used in
contexts where it's important that the message IDs don't collide,
or even that they're increasing, without the test having to explicitly
give specific message IDs when no other fact about them is relevant.

Many call sites that were passing explicit message IDs no longer need
to do so after this change.  We'll clear those out in the next commit.

Author: gnprice

test/example_data.dart

@@ -1,3 +1,5 @@
+import 'dart:math';
+
 import 'package:zulip/api/model/events.dart';
 import 'package:zulip/api/model/initial_snapshot.dart';
 import 'package:zulip/api/model/model.dart';
@@ -164,6 +166,20 @@ Map<String, dynamic> _messagePropertiesFromContent(String? content, String? cont
   }
 }
 
+/// A fresh message ID, from a random but always strictly increasing sequence.
+int _nextMessageId() => (_lastMessageId += 1 + Random().nextInt(100));
+int _lastMessageId = 1000;
+
+/// Construct an example stream message.
+///
+/// If the message ID `id` is not given, it will be generated from a random
+/// but increasing sequence, which is shared with [dmMessage].
+/// Use an explicit `id` only if the ID needs to correspond to some other data
+/// in the test, or if the IDs need to increase in a different order from the
+/// calls to [streamMessage] and [dmMessage].
+///
+/// See also:
+///  * [dmMessage], to construct an example direct message.
 StreamMessage streamMessage({
   int? id,
   User? sender,
@@ -190,7 +206,7 @@ StreamMessage streamMessage({
     'stream_id': effectiveStream.streamId,
     'reactions': reactions == null ? [] : Reactions(reactions),
     'flags': flags ?? [],
-    'id': id ?? 1234567, // TODO generate example IDs
+    'id': id ?? _nextMessageId(),
     'last_edit_timestamp': lastEditTimestamp,
     'subject': topic ?? 'example topic',
     'timestamp': timestamp ?? 1678139636,
@@ -200,6 +216,12 @@ StreamMessage streamMessage({
 
 /// Construct an example direct message.
 ///
+/// If the message ID `id` is not given, it will be generated from a random
+/// but increasing sequence, which is shared with [streamMessage].
+/// Use an explicit `id` only if the ID needs to correspond to some other data
+/// in the test, or if the IDs need to increase in a different order from the
+/// calls to [streamMessage] and [dmMessage].
+///
 /// See also:
 ///  * [streamMessage], to construct an example stream message.
 DmMessage dmMessage({
@@ -222,7 +244,7 @@ DmMessage dmMessage({
       .toList(growable: false),
     'reactions': [],
     'flags': flags ?? [],
-    'id': id ?? 1234567, // TODO generate example IDs
+    'id': id ?? _nextMessageId(),
     'last_edit_timestamp': lastEditTimestamp,
     'subject': '',
     'timestamp': timestamp ?? 1678139636,