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 paths that expose flag results. Confirm how the engine returns the evaluation reason, then ensure the listed reasons are surfaced in both areas; done means SDK API and local-evaluation responses expose Environment Default, Segment Override, and Identity Override.

索引モデルが issue の本文から書いたものです。