message: Add skeleton of handling a move UpdateMessageEvent

The way the Zulip server API expresses whether there was a move,
and if so what kind of move, is a bit quirky.  This change adds
logic to interpret that information into a more canonical form.

The logic here is translated from what we have in zulip-mobile:
  https://github.com/zulip/zulip-mobile/blob/e352f563e/src/api/misc.js#L26

This doesn't yet do anything to actually update our data structures,
and it's an NFC change for release builds.  The only behavior change
is new asserts that fire when the event from the server is malformed.

lib/model/message.dart

@@ -1,5 +1,6 @@
 import '../api/model/events.dart';
 import '../api/model/model.dart';
+import '../log.dart';
 import 'message_list.dart';
 
 /// The portion of [PerAccountStore] for messages and message lists.
@@ -96,8 +97,7 @@ class MessageStoreImpl with MessageStore {
     assert(event.messageIds.contains(event.messageId), "See https://github.com/zulip/zulip-flutter/pull/753#discussion_r1649463633");
     _handleUpdateMessageEventTimestamp(event);
     _handleUpdateMessageEventContent(event);
-    // TODO(#150): Handle message moves.  The views' recipient headers
-    //   may need updating, and consequently showSender too.
+    _handleUpdateMessageEventMove(event);
     for (final view in _messageListViews) {
       view.notifyListenersIfAnyMessagePresent(event.messageIds);
     }
@@ -139,6 +139,41 @@ class MessageStoreImpl with MessageStore {
     }
   }
 
+  void _handleUpdateMessageEventMove(UpdateMessageEvent event) {
+    // The interaction between the fields of these events are a bit tricky.
+    // For reference, see: https://zulip.com/api/get-events#update_message
+
+    if (event.origTopic == null) {
+      // There was no move.
+      assert(() {
+        if (event.newStreamId != null && event.origStreamId != null
+            && event.newStreamId != event.origStreamId) {
+          // This should be impossible; `orig_subject` (aka origTopic) is
+          // documented to be present when either the stream or topic changed.
+          debugLog('Malformed UpdateMessageEvent: stream move but no origTopic'); // TODO(log)
+        }
+        return true;
+      }());
+      return;
+    }
+
+    if (event.newTopic == null) {
+      // The `subject` field (aka newTopic) is documented to be present on moves.
+      assert(debugLog('Malformed UpdateMessageEvent: move but no newTopic')); // TODO(log)
+      return;
+    }
+    if (event.origStreamId == null) {
+      // The `stream_id` field (aka origStreamId) is documented to be present on moves.
+      assert(debugLog('Malformed UpdateMessageEvent: move but no origStreamId')); // TODO(log)
+      return;
+    }
+
+    // final newStreamId = event.newStreamId; // null if topic-only move
+    // final newTopic = event.newTopic!;
+    // TODO(#150): Handle message moves.  The views' recipient headers
+    //   may need updating, and consequently showSender too.
+  }
+
   void handleDeleteMessageEvent(DeleteMessageEvent event) {
     // TODO handle DeleteMessageEvent, particularly in MessageListView
   }