I've updated the `SessionMemory` interface from an `abc.ABC` to a `typing.Protocol`. I've also marked it as `@runtime_checkable` to ensure that `isinstance` checks continue to work as expected.

This change provides more flexibility for implementing custom memory solutions, allowing for structural subtyping (duck typing) while still enabling runtime type verification.

The `SQLiteSessionMemory` implementation and existing type hints and checks in the `Agent` and `Runner` classes remain compatible with this change. Unit tests and documentation have been verified and updated where necessary to reflect `SessionMemory` as a Protocol.