KIP-275 — Indicate "isClosing" in the SinkTaskContext

Discarded Connect

KIP-275 proposed adding an `isClosing()` flag to `SinkTaskContext` to signal to a sink task that the worker is shutting down, enabling the task to flush and commit a partial batch before termination. Without this signal, tasks that accumulate large in-memory batches (e.g., S3 connector) lose all buffered records whenever a task restart or rebalance occurs mid-batch.

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