Update Kagi, You.com, and Tavily search providers to current APIs

[tpae]

Summary

Documentation review of all 8 bundled search providers (prompted by a user report of a 401 when pasting a Kagi API key into Osaurus 0.22.0). Exa, Parallel, Brave Search API, Serper, and Google CSE are current; three providers needed updates:

• Kagi (root cause of the reported 401): migrated from the legacy v0 API (GET /api/v0/search, Authorization: Bot) to v1 — POST https://kagi.com/api/v1/search with Authorization: Bearer, JSON body (query, workflow of search/news, limit, page), and response mapping switched from flat data[] + t == 0 filter to the v1 buckets data.search / data.news. Keys generated in Kagi's new API portal are v1 keys and fail v0 with 401. Setup instructions, signup URL, and pricing note now point at the new portal (kagi.com/api, pay-per-use).
• You.com: migrated the two legacy endpoints (api.ydc-index.io/search with hits[], /news with news.results[]) to the unified GET https://ydc-index.io/v1/search; params renamed to count/freshness; responses map results.web / results.news with the description|snippets fallback and page_age.
• Tavily: API key moved from the legacy api_key body field to the Authorization: Bearer header per current docs.

Also adds the four new Kagi UI strings to Localizable.xcstrings (zh-Hans/zh-Hant translated, de/ko/ru marked needs-review).

Test plan

• Targeted suites pass (67 tests): DeclarativeSearchBackendTests, SearchProviderConfigurationTests, SearchEngineCascadeTests, WebSearchToolTests — including new cases for Kagi v1 request building (Bearer/POST/workflow), bucketed response parsing, and the You.com unified endpoint
• swift build --package-path Packages/OsaurusCore succeeds
• scripts/i18n/check.sh passes
• Live smoke test with invalid keys: Kagi v1 accepts the request shape (general.invalid_token error, no schema rejection), You.com returns 403, Tavily returns 401
• End-to-end retest with a real (paid) Kagi v1 key — needs the reporter or a teammate with API credit