Feat/epoch poll

[NikhilSharmaWe]

Issue Addressed

Which issue # does this PR address?
#8457

Proposed Changes

The change modifies the duties service in validator_client/validator_services/src/duties_service.rs to use duration_to_next_epoch() instead of duration_to_next_slot(), while maintaining the same safety guarantees through the existing dependent_root check mechanism.

[dapplion]

🤖 This breaks block production for slots 1–31 of every epoch. BlockServiceNotification is only emitted inside poll_beacon_proposers (duties_service.rs:1644, :1729), and BlockService::do_update requires notification.slot == now() (block_service.rs:269). With the loop now firing only at epoch boundaries, proposers assigned to non-slot-0 slots are never notified.

Per @michaelsproul's note on #8457, the slot loop should stay; only the BN fetch cadence should change (and ideally fetch current_epoch + 1 post-Fulu — this PR never actually uses proposer_lookahead).