fix(cloudevent): honor target-protocol for Kafka ce_ prefix

[arimu1]

Summary

• Restores MessageUtils.TARGET_PROTOCOL and resolves the target-protocol header (case-insensitive) before inferring protocol from kafka_* / amqp headers in CloudEventMessageUtils.extractTargetProtocol.
• Merges input and result headers in CloudEventsFunctionInvocationHelper.postProcessResult when choosing the output Cloud Event attribute prefix.
• Adds a unit test covering AMQP-sourced messages with target-protocol: kafka (cross-binder scenario from ce_ prefix is no longer applied automatically for Cloud Events sent to Kafka #1461).

Fixes gh-1461

Context

Since gh-1294 / spring-cloud/spring-cloud-stream#2222, Spring Cloud Stream no longer sets target-protocol on messages, and prefix detection falls back to binder-specific headers on the input message. For AMQP → Kafka flows (e.g. function-sample-cloudevent-stream), that yields cloudEvents: on Kafka instead of ce_.

This change restores the function-side support for target-protocol. Spring Cloud Stream should restore setting that header on function input (see commented lines in FunctionConfiguration.FunctionWrapper#setHeadersIfNeeded and the reactive targetProtocolEnhancer) so end-to-end behavior matches the sample README again.

Test plan

• ./mvnw -pl spring-cloud-function-context -am test
• ./mvnw -f spring-cloud-function-samples/function-sample-cloudevent-stream/pom.xml test (module not in parent reactor; requires Rabbit/Kafka on localhost; DemoApplicationTests#test is currently commented out)
• After stream companion change: enable DemoApplicationTests#test and verify Kafka headers use ce_ prefix

[DidierLoiseau]

I’m confused, this fix would require users to set the target-protocol explicitly to work properly, right? Or is it your intent to also revert the changes from spring-cloud/spring-cloud-stream#2222?

[DidierLoiseau]

@olegz this does not solve #1461, it’s only a workaround that allows setting target-protocol when you don’t set any other kafka_ header.

Could you thus reopen #1461?

p.s. for some reason, I’m no longer able to run the test in function-sample-cloudevent-stream today:

• the spring-cloud-function-deployer 5.1.0-SNAPSHOT dependency no longer exists
• even with 5.0.0-M2 of the former + updating spring-cloud-stream dependencies to 5.1.0-SNAPSHOT, it fails with class [B cannot be cast to class io.spring.cloudevent.Person