Python: Investigate moving durableagentstate serialization to SerializationMixin

Currently we have duplicated code for DurableAgentState serialization and `SerializationMixin` since the latter does not completely support all cases needed by Durable Agent State.

Opening this task to investigate if we can unify the 2 by adding support for all custom logic to SerializationMixin.

No files, tests, or entry points are named. Start by locating DurableAgentState serialization and SerializationMixin, then compare the custom cases each supports. Done means determining whether the behaviors can be unified and documenting or testing the required SerializationMixin support.

Written by the indexing model from the issue text.