fix(app): preserve plugin-rendered replies when collapsing turns (#8811)

* fix(app): preserve plugin-rendered replies when collapsing turns

* test(app): exercise plugin reply folding in the browser

packages/app/src/plugins/timeline/projection.test.ts

@@ -98,6 +98,8 @@ describe("plugin timeline projection", () => {
       {
         kind: "plugin",
         id: "inline-thinking/thought-1/0",
+        sourceId: "thought-1",
+        sourceKind: "thought",
         data: { text: "First", phase: "streaming" },
       },
     ]);

packages/app/src/plugins/timeline/projection.ts

@@ -106,6 +106,7 @@ function transformSourceItem(
       kind: "plugin",
       id: `${pluginItem.pluginId}/${pluginItem.id}`,
       sourceId: item.id,
+      sourceKind: item.kind,
       timestamp: item.timestamp,
       pluginId: pluginItem.pluginId,
       pluginItemId: pluginItem.id,