FEATURE: add a `.topic` attribute to transformedPost (#25757)

During extensibility when we add post menu buttons we very much want access
to the topic.

The transformer does not include a `topic` attribute due to historical reasons.

Given we are going to move away from transforming long term and need to give
plugins access to topic when they are adding buttons, just add the extra
property

app/assets/javascripts/discourse/app/lib/transform-post.js

@@ -117,6 +117,7 @@ export default function transformPost(
 
   const createdBy = details.created_by || {};
 
+  postAtts.topic = topic;
   postAtts.topicId = topic.id;
   postAtts.topicOwner = createdBy.id === post.user_id;
   postAtts.topicCreatedById = createdBy.id;

app/assets/javascripts/discourse/tests/integration/components/widgets/post-stream-test.js

@@ -2,8 +2,10 @@ import { getOwner } from "@ember/application";
 import { render } from "@ember/test-helpers";
 import { hbs } from "ember-cli-htmlbars";
 import { module, test } from "qunit";
+import { withPluginApi } from "discourse/lib/plugin-api";
 import { setupRenderingTest } from "discourse/tests/helpers/component-test";
 import { count } from "discourse/tests/helpers/qunit-helpers";
+import { resetPostMenuExtraButtons } from "discourse/widgets/post-menu";
 
 function postStreamTest(name, attrs) {
   test(name, async function (assert) {
@@ -17,9 +19,64 @@ function postStreamTest(name, attrs) {
   });
 }
 
+let lastTransformedPost = null;
+
 module("Integration | Component | Widget | post-stream", function (hooks) {
   setupRenderingTest(hooks);
 
+  hooks.afterEach(function () {
+    resetPostMenuExtraButtons();
+  });
+
+  postStreamTest("extensibility", {
+    posts() {
+      withPluginApi("0.14.0", (api) => {
+        api.addPostMenuButton("coffee", (transformedPost) => {
+          lastTransformedPost = transformedPost;
+          return {
+            action: "drinkCoffee",
+            icon: "coffee",
+            className: "hot-coffee",
+            title: "coffee.title",
+            position: "first",
+          };
+        });
+      });
+
+      const store = getOwner(this).lookup("service:store");
+      const topic = store.createRecord("topic");
+      topic.set("details.created_by", { id: 123 });
+      topic.set("id", 1234);
+
+      return [
+        store.createRecord("post", {
+          topic,
+          id: 1,
+          post_number: 1,
+          user_id: 123,
+          primary_group_name: "trout",
+          avatar_template: "/images/avatar.png",
+        }),
+      ];
+    },
+
+    test(assert) {
+      assert.strictEqual(count(".post-stream"), 1);
+      assert.strictEqual(count(".topic-post"), 1, "renders all posts");
+      assert.notStrictEqual(lastTransformedPost, null, "it transforms posts");
+      assert.strictEqual(
+        lastTransformedPost.topic.id,
+        1234,
+        "it also transforms the topic"
+      );
+      assert.strictEqual(
+        count(".actions .extra-buttons .hot-coffee"),
+        1,
+        "should have the extended button"
+      );
+    },
+  });
+
   postStreamTest("basics", {
     posts() {
       const site = getOwner(this).lookup("service:site");