0.6.28

## What's Changed
* feat: Add model to context length mapping for gpt-4o-mini by @kk-src in https://github.com/letta-ai/letta/pull/2442
* feat: add new providers (bedrock, deepseek) + bugfixes by @sarahwooders in https://github.com/letta-ai/letta/pull/2445


**Full Changelog**: https://github.com/letta-ai/letta/compare/0.6.27...0.6.28