Structured CloudEvent fails to convert to `io.cloudevents.CloudEvent` in the functional model (data lost / "Unknown encoding")

Describe the bug

When a structured-mode CloudEvent (Content-Type: application/cloudevents+json, whole envelope in the body) is consumed by a function whose target type is the CNCF io.cloudevents.CloudEvent, message conversion fails.

CloudEventMessageUtils.toCanonical(...) transforms the structured message into a binary-shaped message (data extracted to the payload, attributes moved to ce-* headers), but:

1. it does not reset the contentType header — the outgoing message keeps application/cloudevents+json even though the payload is no longer the serialized envelope; and
2. it leaves the extracted data as a deserialized Map, not serialized bytes.

Downstream, the CNCF io.cloudevents.spring.messaging.CloudEventMessageConverter re-decides structured-vs-binary purely from contentType. Because the stale application/cloudevents+json remains, it takes the structured reader and tries to re-parse the (already extracted) payload as the envelope:

java.lang.IllegalArgumentException: argument "src" is null
   at io.cloudevents.jackson.JsonFormat.deserialize(JsonFormat.java:158)
   at io.cloudevents.spring.messaging.CloudEventMessageConverter.fromMessage(CloudEventMessageConverter.java:45)
   at org.springframework.cloud.function.context.config.SmartCompositeMessageConverter.fromMessage(...)
   at ...SimpleFunctionRegistry$FunctionInvocationWrapper.convertInputMessageIfNecessary(...)

and 

io.cloudevents.rw.CloudEventRWException: Could not parse. Unknown encoding. Invalid content type or spec version
	at io.cloudevents.core.message.impl.MessageUtils.parseStructuredOrBinaryMessage(MessageUtils.java:69)
	at io.cloudevents.spring.messaging.CloudEventMessageConverter.createMessageReader(CloudEventMessageConverter.java:60)
	at io.cloudevents.spring.messaging.CloudEventMessageConverter.fromMessage(CloudEventMessageConverter.java:45)
	at org.springframework.cloud.function.context.config.SmartCompositeMessageConverter.fromMessage(SmartCompositeMessageConverter.java:137)
	at org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.convertInputMessageIfNecessary(SimpleFunctionRegistry.java:1488)

Even after the contentType is corrected so the binary reader is selected, a second symptom remains: the CNCF binary reader only reads byte[]/String payloads (CloudEventMessageConverter#getBinaryData returns null for anything else), so the Map payload is dropped and the delivered cloudEvent.getData() is null while the attributes are present.

This only affects the io.cloudevents.CloudEvent target type. A plain POJO target works, because Spring's own message converters can bind a Map payload to a POJO — which is why the existing CloudEventFunctionTests#testStructuredPojoToPojoDefaultOutputAttributeProvider passes and this case is not covered.

Affected versions

• spring-cloud-function-context:4.3.x. CloudEventMessageUtils.toCanonical / buildBinaryMessageFromStructuredMap are byte-identical across these.
• io.cloudevents:cloudevents-spring: 2.2.1 (the optional dependency declared by spring-cloud-function-context);
• Java: 21.

Sample CloudEvent (structured mode)

Content-Type: application/cloudevents+json, body:

{
  "specversion" : "1.0",
  "type" : "org.springframework",
  "source" : "https://spring.io/",
  "id" : "A234-1234-1234",
  "datacontenttype" : "application/json",
  "data" : {
    "version" : "1.0",
    "releaseName" : "Spring Framework",
    "releaseDate" : "24-03-2004"
  }
}

Steps to reproduce

Minimal function with an io.cloudevents.CloudEvent input type (the cloudevents-spring converter is auto-registered by ContextFunctionCatalogAutoConfiguration$CloudEventsMessageConverterConfiguration):

@EnableAutoConfiguration
@Configuration
static class TestConfiguration {
    @Bean
    Function<Message<CloudEvent>, Message<CloudEvent>> echoCloudEvent() {
        return Function.identity();
    }
}

@Test
void structuredToCloudEventTypedPayload() {
    String payload = /* the structured JSON envelope above */;

    Function<Object, Object> function = context.getBean(FunctionCatalog.class).lookup("echoCloudEvent");

    Message<String> inputMessage = MessageBuilder
            .withPayload(payload)
            .setHeader(MessageHeaders.CONTENT_TYPE, "application/cloudevents+json")
            .build();

    // structured mode: attributes are in the body, not the headers
    assertThat(CloudEventMessageUtils.isCloudEvent(inputMessage)).isFalse();

    Message<CloudEvent> result = (Message<CloudEvent>) function.apply(inputMessage); // throws
}

Actual behavior

function.apply(...) throws CloudEventRWException: Could not parse. Unknown encoding during input conversion. (If contentType is corrected in isolation, the call succeeds but the delivered CloudEvent has getData() == null.)

Expected behavior

The function receives a fully-populated io.cloudevents.CloudEvent — correct specversion, type, source, id, datacontenttype, and non-null data — consistent with how the same structured event is handled when the target type is a POJO.

Root cause

In CloudEventMessageUtils.buildBinaryMessageFromStructuredMap(...), after extracting data and moving attributes to ce-* headers, the original headers are copied back onto the outgoing message — re-applying contentType=application/cloudevents+json — and the data remains a Map. The binary-mode branch of toCanonical already resets contentType to the data content-type; the structured branch does not, and it does not serialize the data.

Proposed fix

In buildBinaryMessageFromStructuredMap, after building the binary-mode message:

1. reset MessageHeaders.CONTENT_TYPE to the data content-type (mirroring the binary-mode branch), and
2. re-serialize the extracted data to bytes using the data content-type when it is not already byte[]/String.

With both applied, the structured event converts correctly to io.cloudevents.CloudEvent (attributes + data), and all existing CloudEvent tests still pass. Happy to open a PR with a CloudEventFunctionTests case covering the io.cloudevents.CloudEvent target type.

Comienza en CloudEventMessageUtils.buildBinaryMessageFromStructuredMap, que el informe identifica como el encargado de reconstruir eventos estructurados, y revisa la rama binaria de toCanonical para comparar. Añade cobertura en CloudEventFunctionTests para un evento estructurado cuyo destino sea io.cloudevents.CloudEvent y, después, ejecuta las pruebas de CloudEvent. La tarea estará terminada cuando la conversión tenga éxito con los atributos esperados y data no sea null.