Epic: Add granular trait control for client-side SDKs

The Edge API uses client-sent traits for segment evaluation even when `allow_client_traits` is disabled — a security gap that lets untrusted clients influence segment matching. The Core API correctly drops these traits.

Rather than silently changing evaluation behaviour for ~2% of paid SaaS customers who have this toggle disabled, we're adding a new environment-level field `allow_client_traits_in_evaluation` with a grandfathering SaaS-only migration so existing environments keep working while new environments get correct behaviour.

The Environment settings UI should reflect the new field and include a section that allows to define a policy in regards to traits sent over by client-side SDKs:

- The existing allow_client_traits should be controlled by the "Persist" toggle.
- The new allow_client_traits_in_evaluation field should be controlled by "Use in evaluation" toggle.

### References

- Original report: #6739
- Edge API issue: https://github.com/Flagsmith/edge-api/issues/546
- Docs/frontend PR (on hold): #6782
- [Slack thread](https://flagsmith.slack.com/archives/C06P8KV2CC8/p1772126962203039) with CS team discussion