Add event compaction to the Python SDK

This is a pure feature request.

Our current design transparently stores each run to a database as it gets processed by a Python server. At present, we have a homegrown, limited version of event compaction, based on the draft serialization spec. The client then has a way to request state and message snapshot events from the backend for a given thread ID over a standard agi-ui endpoint.

In the short term, we will probably implement an internal direct port of https://github.com/ag-ui-protocol/ag-ui/blob/09edea51dd6b728173dcdeb88c05f779d76e754f/sdks/typescript/packages/client/src/compact/compact.ts to python, but in the longer term, it seems like it might be useful to others to have access to an upstream-maintained implementation of `compactEvents`.