message edit: Move with_first_message_id to message edit.js.

Prep commit so that the function first_message_id can be used
as an exported function for marking resolved topic as unresolved.

Related to #20584.

static/js/message_edit.js

@@ -1099,3 +1099,40 @@ export function move_topic_containing_message_to_stream(
         },
     });
 }
+
+export function with_first_message_id(stream_id, topic_name, success_cb, error_cb) {
+    // The API endpoint for editing messages to change their
+    // content, topic, or stream requires a message ID.
+    //
+    // Because we don't have full data in the browser client, it's
+    // possible that we might display a topic in the left sidebar
+    // (and thus expose the UI for moving its topic to another
+    // stream) without having a message ID that is definitely
+    // within the topic.  (The comments in stream_topic_history.js
+    // discuss the tricky issues around message deletion that are
+    // involved here).
+    //
+    // To ensure this option works reliably at a small latency
+    // cost for a rare operation, we just ask the server for the
+    // latest message ID in the topic.
+    const data = {
+        anchor: "newest",
+        num_before: 1,
+        num_after: 0,
+        narrow: JSON.stringify([
+            {operator: "stream", operand: stream_id},
+            {operator: "topic", operand: topic_name},
+        ]),
+    };
+
+    channel.get({
+        url: "/json/messages",
+        data,
+        idempotent: true,
+        success(data) {
+            const message_id = data.messages[0].id;
+            success_cb(message_id);
+        },
+        error_cb,
+    });
+}