descriptions,search: use `typed: strict` and `T::Enum`

## Summary

- Add `Descriptions::SearchField` enum with `Name`, `Description`, and
  `Either` values to replace symbol-based field parameter
- Upgrade `descriptions.rb` and `search.rb` to `typed: strict`
- Add Sorbet type signatures to all methods in both files
- Use exhaustive case matching with `T.absurd` for type safety
- Remove redundant `cache_store_hash` parameter by using `is_a?` check

Contributes to #17297.