send [nfc]: Always pass readBySender true

I think we're always going to want this parameter to be true
for all the messages we send from the app.

In the API bindings themselves, i.e. in lib/api/ , I generally
prefer not to hard-code this sort of thing.  That lets the API
bindings be a faithful reflection of the Zulip Server API, with
a minimum of ad-hoc specializations for the needs of our app.
So that's why the `sendMessage` binding just takes the parameter
from its caller.

But this method PerAccountStore.sendMessage is already something
specific to our app rather than an API binding; it doesn't do much
now beyond wrap the API, but the reason it exists is to be the
future home of our "outbox" logic.  That means (a) recording the
message locally so that we can display it immediately, (b) possibly
retrying the send, and (c) if the send fails or times out, recording
that fact so that our UI can present the user with that information
and the option to retry.

So that makes it a natural home for encapsulating any other logic
that this app will always want when sending a message.
In particular, passing this parameter.

lib/model/store.dart

@@ -377,13 +377,13 @@ class PerAccountStore extends ChangeNotifier with StreamStore {
     }
   }
 
-  Future<void> sendMessage({required MessageDestination destination, required String content, bool? readBySender}) {
+  Future<void> sendMessage({required MessageDestination destination, required String content}) {
     // TODO implement outbox; see design at
     //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.23M3881.20Sending.20outbox.20messages.20is.20fraught.20with.20issues/near/1405739
     return _apiSendMessage(connection,
       destination: destination,
       content: content,
-      readBySender: readBySender,
+      readBySender: true,
     );
   }
 

lib/widgets/compose_box.dart

@@ -733,7 +733,7 @@ class _SendButtonState extends State<_SendButton> {
 
     final store = PerAccountStoreWidget.of(context);
     final content = widget.contentController.textNormalized;
-    store.sendMessage(destination: widget.getDestination(), content: content, readBySender: true);
+    store.sendMessage(destination: widget.getDestination(), content: content);
 
     widget.contentController.clear();
   }

test/model/store_test.dart

@@ -4,6 +4,7 @@ import 'package:checks/checks.dart';
 import 'package:flutter/foundation.dart';
 import 'package:http/http.dart' as http;
 import 'package:test/scaffolding.dart';
+import 'package:zulip/api/route/messages.dart';
 import 'package:zulip/model/store.dart';
 import 'package:zulip/notifications.dart';
 
@@ -108,6 +109,28 @@ void main() {
     check(completers(1)).length.equals(1);
   });
 
+  group('PerAccountStore.sendMessage', () {
+    test('smoke', () async {
+      final store = eg.store();
+      final connection = store.connection as FakeApiConnection;
+      final stream = eg.stream();
+      connection.prepare(json: SendMessageResult(id: 12345).toJson());
+      await store.sendMessage(
+        destination: StreamDestination(stream.streamId, 'world'),
+        content: 'hello');
+      check(connection.lastRequest).isA<http.Request>()
+        ..method.equals('POST')
+        ..url.path.equals('/api/v1/messages')
+        ..bodyFields.deepEquals({
+          'type': 'stream',
+          'to': stream.streamId.toString(),
+          'topic': 'world',
+          'content': 'hello',
+          'read_by_sender': 'true',
+        });
+    });
+  });
+
   group('UpdateMachine.registerNotificationToken', () {
     late UpdateMachine updateMachine;
     late FakeApiConnection connection;