feat: update AI Search type definitions for V5 namespace binding spec

Aligns ai-search.d.ts with the V5 binding spec (path-based namespaces).

Key changes:
- Rename AiSearchAccountService -> AiSearchNamespace (namespace-scoped, not account-scoped)
- Rename AiSearchInstanceService -> AiSearchInstance
- Add delete(name) to AiSearchNamespace (namespace-level deletion)
- Remove delete() from AiSearchInstance (moved to namespace level)
- Add update(), stats(), items, jobs to AiSearchInstance
- Add sub-service types: AiSearchItems, AiSearchItem, AiSearchJobs, AiSearchJob
- Add item/job metadata types and request/response types
- Remove AiSearchNameNotSetError (V4 concept, namespace always present in V5)
- Update Ai.aiSearch() return type and JSDoc
- Update AutoRAG @see references

Author: G4brym