KIP-1328 — Dead Letter Queue Support for DSL Dropped Records

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Adds ProcessingContext#writeToDlq and a DroppedRecordException so DSL operators that silently drop records — null keys or values in aggregate/count/reduce, late records past the grace period in windowed aggregations, null join keys in stream-table and foreign-key joins, null keys in toTable — route them to the KIP-1034 dead letter queue with the standard error headers, while still recording the dropped-records sensor. Those drops were only observable as a WARN log and a counter, with no way to capture the payloads for analysis or reprocessing, unlike deserialization and processing errors which already had a DLQ path.

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