fetching leader schedule in sentinel for local validator instance is unusable

[karl-dz]

Cannot perform local testing with the current logic of PreviousEpochSlots: https://github.com/doublezerofoundation/doublezero-offchain/blob/51b700f81c01e0152cc5855079243c5779c1a0d2/crates/sentinel/src/client/solana.rs#L368-L379.

Because a local validator does not usually persist for more than an epoch, the next value will always return None. This causes issues in CI with requesting Solana validator access like in this PR.