fix: avoid redundant feature reloads during sticky bucket refresh

[vazarkevych]

What

_get_sticky_bucket_attributes() called _get_eval_context() once per sticky-bucket identifier attribute. _get_eval_context() runs _ensure_fresh_features() → load_features(), so on the set_attributes() path (where the _is_updating_features guard is not set) this triggered N redundant feature reloads per refresh - one per identifier attribute - each re-running set_features() and the sticky-bucket service lookup.

How

• Split context assembly into a side-effect-free _build_eval_context().
• _get_eval_context() now = _ensure_fresh_features() + _build_eval_context(), so the main eval paths (run / eval_feature) keep lazy refresh unchanged.
• _get_sticky_bucket_attributes() builds the context once via _build_eval_context() → no reloads during refresh.

Also hardened eval_prereqs(): a non-dict prerequisite condition is now skipped instead of crashing in evalCondition(...).items().