Allow get_beacon_proposer_index at next epoch

[michaelsproul]

Proposed Changes

Continued cleanup of proposer index logic, this PR allows get_beacon_proposer_index to be called at the next epoch (post-Fulu), bringing it in line with get_beacon_proposer_indices after the changes in:

• More proposer shuffling cleanup #8130

This is a safe change for Fulu due to the newly introduced lookahead.

Additional Info

• Needs new/updated tests to cover this newly added case. I think it would be simplest to extend the tests added in More proposer shuffling cleanup #8130, so will wait for that to merge.

This change is non-essential for correctness, as get_beacon_proposer_index is currently only called for the current epoch (state.slot() or block.slot() after state advance), and in tests.