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

Trace the SDK API response and local evaluation entry points, then inspect the flag and response object types used by both paths. Done means the engine's evaluation reason is exposed in SDK flag/response objects for Environment Default, Segment Override, and Identity Override in both modes.

索引モデルが issue の本文から書いたものです。