KIP-594 — Expose output topic names from TopologyTestDriver

Accepted Kafka 2.6 StreamsTesting

Adds a `producedTopicNames()` method to `TopologyTestDriver` that returns all topic names the topology wrote to during a test run, including internal repartition and changelog topics. This enables catch-all assertions that detect unexpected topic writes, which is impossible today without explicitly naming every output topic in each test.

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