address gateway tako search tool feedback (#19466)

## Background

Align the AI SDK Tako Search public contract with the corresponding
Gateway implementation and improve
  guidance for cost-sensitive data options.

  ## Summary

- Remove `relevance_score` and `citation_number` from the public
response types and output-schema
  declarations.
- Preserve runtime passthrough behavior for undeclared response
properties.
- Update the camelCase JSDoc and equivalent snake_case schema
descriptions for data result counts, content
  inclusion, and row limits.
  - Update the example so it no longer references `citation_number`.
  - Add focused runtime and type-level contract tests.
  - Add a patch changeset for `@ai-sdk/gateway`.

  ## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
  - [x] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm
  changeset` in the project root)
  - [x] I have reviewed this pull request (self-review)