ML-188: guard PubSub record updates during edit mode

Add live_action guard to handle_info({:update, record}) in both
CollectionLive.Show and WishlistLive.Show. When the user is editing,
background worker updates are skipped and a warning toast is shown
instead of overwriting the socket. Normal show-mode updates and
mismatched-ID no-ops are unchanged.

6 tests added covering all three code paths.

lib/music_library_web/live_helpers/record_actions.ex

@@ -119,6 +119,10 @@ defmodule MusicLibraryWeb.LiveHelpers.RecordActions do
   @doc """
   Handles a background record update. Returns the updated socket (not wrapped
   in `{:noreply, ...}`) so the caller can pipe additional assigns.
+
+  **Note:** Callers intentionally bypass this function when `live_action == :edit`
+  to prevent background worker updates from overwriting the user's in-progress
+  form edits. See CollectionLive.Show and WishlistLive.Show `handle_info({:update, record})`.
   """
   def handle_record_updated(socket, record) do
     socket