Epic: Add granular trait control for client-side SDKs

[khvn26]

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: Disabling traits persistence prevents traits to be used in segment evaluation #6739
• Edge API issue: https://github.com/Flagsmith/edge-api/issues/546
• Docs/frontend PR (on hold): docs: Clarify allow_client_traits toggle name and behaviour #6782
• Slack thread with CS team discussion