KIP-448 — Add State Stores Unit Test Support to Kafka Streams Test Utils

Discussion StreamsTesting

KIP-448 adds `MockStoreFactory` and mock `KeyValueStoreBuilder`, `WindowStoreBuilder`, and `SessionStoreBuilder` implementations to the `kafka-streams-test-utils` module so developers can build state stores backed by simple in-memory maps for unit testing. Writing unit tests for DSL operators previously required hand-rolling complex EasyMock-based store builder mocks, making test code verbose and fragile.

Details