FEATURE: Add rake task to mark old hashtag format for rebake (#19876)

Since the new hashtag format has been added, we want site
admins to be able to rebake old posts with the old hashtag
format. This can now be done with `rake hashtags:mark_old_format_for_rebake`
which goes and marks posts with the old cooked version of hashtags
in this format for rebake:

```
<a class=\"hashtag\" href=\"/c/ux/14\">#<span>ux</span></a>
```

c.f. https://meta.discourse.org/t/what-rebake-is-required-for-the-new-autocomplete-styling/249642/12

lib/tasks/hashtags.rake

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+desc "Mark posts with the old hashtag cooked format (pre enable_experimental_hashtag_autocomplete) for rebake"
+task "hashtags:mark_old_format_for_rebake" => :environment do
+  # See Post#rebake_old, which is called via the PeriodicalUpdates job
+  # on a schedule.
+  puts "Finding posts matching old format, this could take some time..."
+  posts_to_rebake = Post.where("cooked like '%class=\"hashtag\"%'")
+  STDOUT.puts(
+    "[!] You are about to mark #{posts_to_rebake.count} posts containing hashtags in the old format to rebake. [CTRL+c] to cancel, [ENTER] to continue",
+  )
+  STDIN.gets.chomp if !Rails.env.test?
+  posts_to_rebake.update_all(baked_version: 0)
+  puts "Done, rebakes will happen when periodal updates job runs."
+end

spec/tasks/hashtags_spec.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+RSpec.describe "tasks/hashtags" do
+  before do
+    Rake::Task.clear
+    Discourse::Application.load_tasks
+  end
+
+  describe "hashtag:mark_old_format_for_rebake" do
+    fab!(:category) { Fabricate(:category, slug: "support") }
+
+    before { SiteSetting.enable_experimental_hashtag_autocomplete = false }
+
+    it "sets the baked_version to 0 for matching posts" do
+      post_1 = Fabricate(:post, raw: "This is a cool #support hashtag")
+      post_2 =
+        Fabricate(
+          :post,
+          raw:
+            "Some other thing which will not match <a class=\"hashtag-wow\">some weird custom thing</a>",
+        )
+
+      SiteSetting.enable_experimental_hashtag_autocomplete = true
+      post_3 = Fabricate(:post, raw: "This is a cool #support hashtag")
+      SiteSetting.enable_experimental_hashtag_autocomplete = false
+
+      Rake::Task["hashtags:mark_old_format_for_rebake"].invoke
+
+      [post_1, post_2, post_3].each(&:reload)
+
+      expect(post_1.baked_version).to eq(0)
+      expect(post_2.baked_version).to eq(Post::BAKED_VERSION)
+      expect(post_3.baked_version).to eq(Post::BAKED_VERSION)
+    end
+  end
+end