Register Expression serializer on binder ObjectMapper

[wadhwaroh-lang]

Fixes #3175.

Description

The bindings actuator endpoint can fail when binder extension properties contain SpEL Expression values, such as Rabbit routingKeyExpression or delayExpression.

AbstractMessageChannelBinder already defines an ExpressionSerializer, but the serializer module was created without being registered on the binder ObjectMapper. As a result, the endpoint could still try to serialize Expression internals and hit Jackson self-reference/cycle errors.

This change registers the existing serializer module when rebuilding the binder ObjectMapper, so SpEL expressions are rendered as their expression string.

Verification

• Added coverage for serializing an Expression through the binder ObjectMapper.
• Ran ./mvnw -pl core/spring-cloud-stream -Dtest=AbstractMessageChannelBinderTests test.

[wadhwaroh-lang]

hi @olegz Can you please help review?

[wadhwaroh-lang]

The remaining Build and Verify failure appears unrelated to this change. The workflow fails during the Setup Maven step before tests run:
No valid download found for version 3.8.9
It looks like jvalkeal/setup-maven@v1 cannot find Maven 3.8.9 at https://dlcdn.apache.org/maven/maven-3/.