Initial yt-dlp implementation (#3)

* Added basic runner for yt-dlp commands

* Updated formatting rules

* Added JSON runner; Added test script for CLI interactions

* Reworked yt-dlp runner; added proper test mocks

test/pinchflat/downloader_backends/yt_dlp/video_collection_test.exs

@@ -0,0 +1,44 @@
+defmodule Pinchflat.DownloaderBackends.YtDlp.VideoCollectionTest do
+  use ExUnit.Case, async: true
+  import Mox
+
+  alias Pinchflat.DownloaderBackends.YtDlp.VideoCollection, as: VideoCollection
+
+  @channel_url "https://www.youtube.com/@TheUselessTrials"
+
+  setup :verify_on_exit!
+
+  describe "get_video_ids/2" do
+    test "returns a list of video ids with no blank elements" do
+      expect(CommandRunnerMock, :run, fn _url, _opts -> {:ok, "id1\nid2\n\nid3\n"} end)
+
+      assert {:ok, ["id1", "id2", "id3"]} = VideoCollection.get_video_ids(@channel_url)
+    end
+
+    test "it passes the expected default args" do
+      expect(CommandRunnerMock, :run, fn _url, opts ->
+        assert opts == [:simulate, :skip_download, :get_id]
+
+        {:ok, ""}
+      end)
+
+      assert {:ok, _} = VideoCollection.get_video_ids(@channel_url)
+    end
+
+    test "it passes the expected custom args" do
+      expect(CommandRunnerMock, :run, fn _url, opts ->
+        assert opts == [:custom_arg, :simulate, :skip_download, :get_id]
+
+        {:ok, ""}
+      end)
+
+      assert {:ok, _} = VideoCollection.get_video_ids(@channel_url, [:custom_arg])
+    end
+
+    test "returns the error straight through when the command fails" do
+      expect(CommandRunnerMock, :run, fn _url, _opts -> {:error, "Big issue", 1} end)
+
+      assert {:error, "Big issue", 1} = VideoCollection.get_video_ids(@channel_url)
+    end
+  end
+end