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 6d6b451c47754d9c2c336dab1a39dbc66d7294e7)