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 how SDK API responses and local-evaluation results become flag and response objects, then identify where the engine's evaluation reason is available. Verify that Environment Default, Segment Override, and Identity Override are exposed in both paths, with tests or response checks covering each case.

索引モデルが issue の本文から書いたものです。