#558 Fluent and type-safe testing DSL for kstreamplify-core-test

[AnkurSinhaaa]

Closes #558
Adds a fluent, type-safe Given → When → Then testing DSL to kstreamplify-core-test, available through a new test() method on KafkaStreamsStarterTest. Tests no longer need to declare and wire TestInputTopic/TestOutputTopic fields or read and assert records manually: the DSL reuses TopicWithSerde as the single source of topic and SerDe information and provides typed output assertions (hasExactly, containsKey, containsRecord, containsValue, containsHeader, containsExactly, satisfies, allSatisfy, ...), first-class DLQ assertions that hide the internal KafkaError behind a DlqRecord, state store assertions, multiple input topics via and(...), timestamped records, event-time and wall-clock control that preserves the Kafka Streams distinction between the two, chaining across several output topics, the DLQ and state stores in a single scenario, and failure messages reporting the topic, the expectation and the records actually produced. The change is purely additive — kstreamplify-core is untouched, existing tests and createInputTestTopic(...)/createOutputTestTopic(...) keep working, and driver() is exposed on every stage as an escape hatch to the underlying TopologyTestDriver. It comes with 70 unit tests covering every assertion and failure message over a topology exercising multiple inputs, DLQ routing, headers, a stream-table join, a state store and a wall-clock punctuator, plus a README section documenting the new recommended test style.