refactor: unify web search provider error handling and fix any types (#704)

- Change Tavily and Querit to throw errors instead of silently returning
  empty results, consistent with BoCha. The outer catch in index.ts
  already provides the fallback.
- Replace `any` type annotations with explicit inline types in both
  Tavily and Querit response mapping.

(cherry picked from commit db289a1d96976d3ca9f82be6542cd4a6469aa46d)