Surface engine reasons in SDK response objects

Part of https://github.com/Flagsmith/flagsmith/issues/8232

Surface the evaluation `reason` (Environment Default, Segment Override, Identity Override) returned by the engine in the SDK's flag/response objects.

Has to cover:
- SDK API responses
- Local evaluation

Start by tracing the SDK API response objects and the local evaluation path to see how engine results are represented. Follow the linked parent issue for the expected reason values, and consider the work complete when Environment Default, Segment Override, and Identity Override reasons are exposed in flag/response objects for both evaluation modes.

Vom Indexierungsmodell aus dem Issue-Text verfasst.