compose [nfc]: Pull out method for computing topic for hint text

This method is trivial for now, but provides a home for logic to
be added next.

In particular this separates the computation of how the topic should
appear in the hint text from what the topic should be in the API,
in `destination`.

Author: gnprice

lib/widgets/compose_box.dart

@@ -579,23 +579,31 @@ class _StreamContentInputState extends State<_StreamContentInput> {
     super.dispose();
   }
 
+  /// The topic name to use in the hint text.
+  TopicName _hintTopic() {
+    return TopicName(widget.controller.topic.textNormalized);
+  }
+
   @override
   Widget build(BuildContext context) {
     final store = PerAccountStoreWidget.of(context);
     final zulipLocalizations = ZulipLocalizations.of(context);
+
     final streamName = store.streams[widget.narrow.streamId]?.name
       ?? zulipLocalizations.unknownChannelName;
-    final topic = TopicName(widget.controller.topic.textNormalized);
+    final hintDestination =
+      // No i18n of this use of "#" and ">" string; those are part of how
+      // Zulip expresses channels and topics, not any normal English punctuation,
+      // so don't make sense to translate. See:
+      //   https://github.com/zulip/zulip-flutter/pull/1148#discussion_r1941990585
+      '#$streamName > ${_hintTopic().displayName}';
+
     return _ContentInput(
       narrow: widget.narrow,
-      destination: TopicNarrow(widget.narrow.streamId, topic),
+      destination: TopicNarrow(widget.narrow.streamId,
+        TopicName(widget.controller.topic.textNormalized)),
       controller: widget.controller,
-      hintText: zulipLocalizations.composeBoxChannelContentHint(
-        // No i18n of this use of "#" and ">" string; those are part of how
-        // Zulip expresses channels and topics, not any normal English punctuation,
-        // so don't make sense to translate. See:
-        //   https://github.com/zulip/zulip-flutter/pull/1148#discussion_r1941990585
-        '#$streamName > ${topic.displayName}'));
+      hintText: zulipLocalizations.composeBoxChannelContentHint(hintDestination));
   }
 }