KIP-782 — Expandable batch size in producer

Discussion Producer

Introduces a `batch.max.bytes` producer config that controls the maximum size at which a batch is drained from the accumulator, decoupling it from `batch.size` which governs memory allocation. Currently `batch.size` serves both roles, forcing operators to choose between memory waste (large allocation) and low throughput (small batch), with no way to tune the two concerns independently.

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