feat: add currentVoteEpoch oprf-accountant helper to BillingContract

[fabian1409]

Note

Low Risk
Additive view-only API with no storage or authorization changes; timing/era edge cases are covered by extensive new tests, though incorrect epoch selection would mislead off-chain voters.

Overview
Adds currentVoteEpoch(signer) on BillingContract / IBillingContract: a single read that returns the latest fully elapsed epoch (not the in-flight one), its [epochStart, epochEnd) span, votingWindowEnd, whether that signer has closed a chunked vote (hasVoted), and block.timestamp. Intended so an OPRF billing accountant can decide when/what to vote without extra RPCs.

The era walk mirrors epochWatermarks / _latestClosedEpoch logic but keys off each era’s span start rather than voting-window close, so mid-epoch callers still see the previous elapsed epoch. exists is false only while epoch 0 is still running; alreadyVoted is per-signer and resets after finalizeEpochs prunes transient submitter state.

Foundry coverage adds a dedicated currentVoteEpoch test block (genesis, progression, per-signer votes, incomplete chunks, finalization, timing-era changes, EpochTooLarge, and multi-epoch participation).

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[fabian1409]

approach to billing will be different, so this is obsolete