Add automated integration and E2E tests for Kafka event sourcing. ## Automated tests (CI) - Function with `run.kafka` config deploys successfully - Kafka env vars (`FUNC_TRANSPORT`, `KAFKA_BROKERS`, `KAFKA_TOPIC`, `KAFKA_CONSUMER_GROUP`) are propagated to the container - Validation rejects incomplete Kafka config before deploy - Validation rejects `run.kafka` on non-Go runtimes - End-to-end: deploy a Kafka function, produce a message, verify the function receives and processes a CloudEvent Requires a Kafka broker in the test environment (e.g. Strimzi on kind). ## Testing guide Document how to manually test Kafka functions end-to-end on a local cluster — setting up kind + Knative + Kafka, deploying, producing messages, verifying. Related: - #3938 (E2E testing guide — can be merged into this issue or kept separate) - #3950 (Kafka event sourcing PR)