Callbacks Improvements

LangChain has announced improvements to its callbacks system, which supports logging, tracing, streaming output, and third-party integrations. The updated system now better supports concurrent runs with independent callbacks, tracing of deeply nested trees of LangChain components, and callback handlers scoped to a single request. This is particularly useful for deploying LangChain on a server. Changes include declaring callbacks in constructor args or passing them directly to run/call/apply methods, the addition of runManager argument, and changes to verbose flag functionality. Some breaking changes and deprecations have also been introduced.