KIP-776 — Add Consumer#peek for debugging/tuning

Discussion Consumer

Adds a `Consumer.peek()` method that returns the next batch of records without advancing the fetch position or committed offsets, and makes `poll()` propagate fetch errors rather than silently returning empty results. Currently an empty `poll()` result is ambiguous between genuine unavailability and a fetch failure, and there is no way to inspect records without consuming them.

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