Kafka functions: integration and E2E tests

[aliok]

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:

• Kafka functions: E2E testing guide #3938 (E2E testing guide — can be merged into this issue or kept separate)
• Add Kafka event sourcing for Go functions #3950 (Kafka event sourcing PR)