* feat: introduce unified search system with legacy support
- Added a new `SearchSystem` interface and `SearchSystemFactory` to manage both legacy and v3 search implementations.
- Updated various components to utilize the new factory for creating retrievers and managing indexing operations.
- Introduced `useLegacySearch` setting to toggle between legacy and v3 search systems, enhancing user control over search functionality.
- Refactored existing search-related code to streamline indexing and retrieval processes, improving overall performance and maintainability.
- Updated documentation to reflect changes in search architecture and settings management.
* feat: enhance V3 indexing responses with document count
- Updated the V3SearchIndexer to return the document count after incremental and full indexing operations.
- Modified success messages to include the number of documents indexed, improving user feedback and clarity on indexing results.
* feat: improve error handling and user feedback for indexing operations
- Enhanced the indexing process by adding error handling for both incremental and full indexing operations, providing users with clear notifications in case of failures.
- Updated the `refreshVaultIndex` and `forceReindexVault` functions to log errors and display messages when indexing fails, improving overall user experience and transparency.
- Refactored the main indexing logic in `CopilotPlugin` to ensure proper loading of indexes based on the selected strategy, enhancing functionality and reliability.
* fix: update indexing logic for search system consistency
- Changed comments to clarify that the index loading occurs even when search is disabled.
- Refactored the index loading process to consistently use `SearchSystemFactory`, enhancing code clarity and maintainability.
- Updated the condition for enabling semantic search to account for the new `useLegacySearch` setting, improving search functionality control.
