Stop CLAP-search route test from leaking a divergent module mock

Commit notes

library.ai-search.error.test.ts mock.module-replaced ../services/clap-search with a getSafeClapSearchFailure re-implemented around a local error subclass. bun's mock.module is global and persists across files, so that divergent implementation leaked into clap-search.test.ts and made it classify a real ClapSearchNotReadyError as a generic 503 (expected 200) — the intermittent CI failure. Reuse the real pure helpers from clap-search-policy in the mock and throw the real error class, so the mock can't diverge from production.

Co-Authored-By: Claude Opus 4.8 <[email redacted]>