DEV: Add topic_embed_import_create_args plugin modifier (#26527)

* DEV: Add `topic_embed_import_create_args` plugin modifier

This modifier allows a plugin to change the arguments used when creating
 a new topic for an imported article.

For example: let's say you want to prepend "Imported: " to the title of
every imported topic. You could use this modifier like so:

```ruby
# In your plugin's code
plugin.register_modifier(:topic_embed_import_create_args) do |args|
  args[:title] = "Imported: #{args[:title]}"
  args
end
```

In this example, the modifier is prepending "Imported: " to the `title` in the `create_args` hash. This modified title would then be used when the new topic is created.

app/models/topic_embed.rb

@@ -81,6 +81,17 @@ class TopicEmbed < ActiveRecord::Base
         }
         create_args[:visible] = false if SiteSetting.import_embed_unlisted?
 
+        # always return `args` when using this modifier, e.g:
+        #
+        # plugin.register_modifier(:topic_embed_import_create_args) do |args|
+        #   args[:title] = "MODIFIED: #{args[:title]}"
+        #
+        #   args # returning args is important to prevent errors
+        # end
+        create_args =
+          DiscoursePluginRegistry.apply_modifier(:topic_embed_import_create_args, create_args) ||
+            create_args
+
         post = PostCreator.create(user, create_args)
         post.topic.topic_embed.update!(embed_content_cache: original_contents)
       end

spec/models/topic_embed_spec.rb

@@ -245,6 +245,46 @@ RSpec.describe TopicEmbed do
           )
         expect(imported_post.cooked).to match(/onebox|iframe/)
       end
+
+      describe "topic_embed_import_create_args modifier" do
+        after { DiscoursePluginRegistry.clear_modifiers! }
+
+        it "can alter the args used to create the topic" do
+          plugin = Plugin::Instance.new
+          plugin.register_modifier(:topic_embed_import_create_args) do |args|
+            args[:title] = "MODIFIED: #{args[:title]}"
+
+            args
+          end
+
+          Jobs.run_immediately!
+          imported_post =
+            TopicEmbed.import(
+              user,
+              "http://eviltrout.com/abcd",
+              title,
+              "some random content",
+              category_id: category.id,
+            )
+          expect(imported_post.topic.title).to eq("MODIFIED: #{title}")
+        end
+
+        it "will revert to defaults if the modifier returns nil" do
+          plugin = Plugin::Instance.new
+          plugin.register_modifier(:topic_embed_import_create_args) { |args| nil }
+
+          Jobs.run_immediately!
+          imported_post =
+            TopicEmbed.import(
+              user,
+              "http://eviltrout.com/abcd",
+              title,
+              "some random content",
+              category_id: category.id,
+            )
+          expect(imported_post.topic.title).to eq(title)
+        end
+      end
     end
 
     context "when post creation supports markdown rendering" do
@@ -554,8 +594,8 @@ RSpec.describe TopicEmbed do
       url = "https://somesource.com"
 
       contents = <<~HTML
-      hello world new post <a href="mailto:somemail@somewhere.org>">hello</a>
-      some image <img src="https:/><invalidimagesrc/">
+        hello world new post <a href="mailto:somemail@somewhere.org>">hello</a>
+        some image <img src="https:/><invalidimagesrc/">
       HTML
 
       raw = TopicEmbed.absolutize_urls(url, contents)