KIP-485 — Make topic optional when using through() operations in DSL

Discussion Kafka 2.6 Streams

Proposes making the intermediate repartition topic optional in Kafka Streams DSL's `through()` operation by allowing the framework to auto-infer when repartitioning is needed after a key-changing operation like `map()` before a `transform()`. Without this, calling `transform()` after a key-changing operation silently skips the necessary repartition step, producing incorrect results when downstream processing depends on key-based partitioning.

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