KIP-1027 — Add MockFixedKeyProcessorContext and TestFixedKeyRecordFactory

Discussion StreamsTesting

Adds `MockFixedKeyProcessorContext` and `TestFixedKeyRecordFactory` to Kafka Streams test-utils to enable unit testing of `FixedKeyProcessor` topologies introduced in KIP-478/KIP-820. The existing `MockProcessorContext` only supports the general `Processor` API; `FixedKeyProcessor` uses a different context type (`FixedKeyProcessorContext`) which had no test-utils analog, making unit tests for fixed-key topologies impossible without an integration test setup.

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