feat(search): add type inference fallback and cached search index

Add automatic type inference when a type filter returns no matches and

fallback to inferred types or no-type search to surface relevant results.

Build and cache a derived search index to speed suggestions and improve

search quality. Tighten type matching to require exact type equality.

- Add SearchIndex and cache path to CacheManager

- Implement DocService.getSearchIndex(), searchWithFallback(),

  suggestTypes(), getAvailableTypes()

- Update formatSearchResults to display fallback info and suggestions

- Enhance python_docs tool; add suggest_python_doc_types

- Bump package version to 0.2.0

- Update tests and mocks; add tests for search index inference