feat: Add idiomatic Kotlin consumer DSL

[rholshausen]

Summary

• Implements the consumer/kotlin module as a full idiomatic Kotlin DSL for writing consumer-side Pact tests (closes Consumer Kotlin DSL API proposal #1352)
• Adds a DSL with @DslMarker-scoped receivers, lambda builders for pact {}, uponReceiving {}, withRequest {}, willRespondWith {}, and a runConsumerTest helper
• Adds a comprehensive README covering BDD style, declarative style, request/response configuration, body matchers, provider states, pending interactions, and mock server usage
• Adds the same doctest infrastructure used in consumer/ — 29 README code blocks are verified to compile and pass on every build via checkDoctests and testDoctest

Test plan

• ./gradlew :consumer:kotlin:check passes (unit tests, checkDoctests, testDoctest, detekt)
• All 29 doctest stubs compile and run without @Disabled
• checkDoctests verifies stubs stay in sync with the README

Fixes #1352

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