FIX: Cook hashtags in small action posts (#23008)

There is no decorateCooked equivalent for small action posts,
so we need to manually call decorateHashtags when there is a custom
message for small action posts in order for the hashtags to get
their coloured icon/square.

spec/system/hashtag_autocomplete_spec.rb

@@ -149,6 +149,49 @@ describe "Using #hashtag autocompletion to search for and lookup categories and
     end
   end
 
+  it "decorates post small actions with hashtags in the custom message" do
+    post =
+      Fabricate(:small_action, raw: "this is a #cool-cat category and a #cooltag tag", topic: topic)
+    topic_page.visit_topic(topic)
+    expect(topic_page).to have_post_number(post.post_number)
+    expect(find(".small-action-custom-message")["innerHTML"]).to have_tag(
+      "a",
+      with: {
+        class: "hashtag-cooked",
+        href: tag.url,
+        "data-type": "tag",
+        "data-slug": tag.name,
+        "data-id": tag.id,
+        "aria-label": tag.name,
+      },
+    ) do
+      with_tag(
+        "svg",
+        with: {
+          class: "fa d-icon d-icon-tag svg-icon hashtag-color--tag-#{tag.id} svg-string",
+        },
+      ) { with_tag("use", with: { href: "#tag" }) }
+    end
+    expect(find(".small-action-custom-message")["innerHTML"]).to have_tag(
+      "a",
+      with: {
+        class: "hashtag-cooked",
+        href: category.url,
+        "data-type": "category",
+        "data-slug": category.slug,
+        "data-id": category.id,
+        "aria-label": category.name,
+      },
+    ) do
+      with_tag(
+        "span",
+        with: {
+          class: "hashtag-category-badge hashtag-color--category-#{category.id}",
+        },
+      )
+    end
+  end
+
   context "when a user cannot access the category for a hashtag cooked in another post" do
     fab!(:admin) { Fabricate(:admin) }
     fab!(:manager_group) { Fabricate(:group, name: "Managers") }