FIX: "in posts by" user search (#27628)

When visiting a user profile, and then opening the search, there's an option to filter down by posts made by that user.

When clicking that option, it used to pre-fill the "search bar" with "@<username>" to filter down the search.

This restore this behaviour and add a system spec to ensure it doesn't regress.

Context - https://meta.discourse.org/t/in-posts-by-search-option-does-not-work-when-clicked/312916

spec/system/user_page/user_page_search_spec.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+describe "User page search", type: :system do
+  fab!(:user)
+  let(:search_page) { PageObjects::Pages::Search.new }
+
+  it "filters down to the user" do
+    sign_in(user)
+
+    visit("/u/#{user.username}")
+    search_page.click_search_icon
+    search_page.click_in_posts_by_user
+
+    expect(search_page).to have_found_no_results
+    expect(search_page.search_term).to eq("@#{user.username}")
+  end
+end