Merge pull request #4 from vendasta/merge-upstream-2

Merge in upstream changes from tmc/langchaingo

Pre-release v0.1.14-pre.4

- Add prompt caching and reasoning token support (tmc#1394)
- Add comprehensive LLM testing framework
- Update Milvus SDK deprecation notice (tmc#1398)
- Complete basic chat app tutorial documentation (tmc#1399)
- Add working example for basic chat application
- Various test improvements and bug fixes