chore: Update rss recipe to use handlers again, pull out and test RSS parsing

[jsantell]

Summary by cubic

Switches the RSS recipe back to handler-based updates and extracts RSS/Atom parsing into a reusable utility with tests. This simplifies the recipe and makes parsing easier to validate.

• Refactors
  • Moved parseRSSFeed and FeedItem to recipes/rss-utils.ts; added unit tests for DOMParser/XML and parseRSSFeed.
  • Updated recipes/rss.tsx to use handler with Cell; derives from fetchData and appends new items after de-duping by id.
  • Removed inlined parser logic (120+ lines) from the recipe.

[cubic-dev-ai]

No issues found across 3 files