🧹 refactor umbral keyfrag deserialization to be type-safe

[scobru]

🎯 What: The code health issue addressed is the usage of an any cast to bypass type checking for @nucypher/umbral-pre when deserializing KeyFrag, along with an Italian workaround comment.
💡 Why: This improves maintainability and code readability by using the correct, safe type structure provided by the TypeScript compiler and making the code more understandable to English speakers.
✅ Verification: The file tpre.ts was manually patched. A test compilation step inside the relay folder via pnpm tsc --noEmit and running tests in the folder using pnpm run lint and pnpm run test proved that the type change is secure and functionally equivalent.
✨ Result: The codebase is now safer and cleaner because any usage has been removed.

---

PR created automatically by Jules for task 564502972286536523 started by @scobru

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.