Simplify CloudEvent formatting with SDK strategies

Replace custom CloudEvent converter infrastructure with direct
CloudEvents SDK format implementations.

Key changes:
- Replace `FormatStrategy` pattern-based approach with direct
  `EventFormatProvider` integration from CloudEvents SDK
- Remove custom converter classes (`CloudEventMessageConverter`,
  `MessageBinaryMessageReader`, `MessageBuilderMessageWriter`)
- Simplify transformer to use Expression-based configuration for
  all CloudEvent attributes (id, source, type, dataSchema, subject)
- Add validation for required CloudEvent attributes with clear
  error messages when expressions evaluate to null or empty values
- Update documentation to reflect Expression-based API and
  byte[] payload requirement
- Consolidate tests by removing coverage for deleted converter
  infrastructure

Author: cppwfs

build.gradle

@@ -480,6 +480,15 @@ project('spring-integration-cloudevents') {
 
     dependencies {
         api "io.cloudevents:cloudevents-core:$cloudEventsVersion"
+        optionalApi "io.cloudevents:cloudevents-spring:$cloudEventsVersion"
+        optionalApi "io.cloudevents:cloudevents-json-jackson:$cloudEventsVersion"
+
+        optionalApi("io.cloudevents:cloudevents-avro-compact:$cloudEventsVersion") {
+            exclude group: 'org.apache.avro', module: 'avro'
+        }
+        optionalApi "org.apache.avro:avro:$avroVersion"
+        optionalApi "io.cloudevents:cloudevents-xml:$cloudEventsVersion"
+        testImplementation 'tools.jackson.core:jackson-databind'
     }
 }
 

spring-integration-cloudevents/src/main/java/org/springframework/integration/cloudevents/package-info.java

@@ -0,0 +1,20 @@
+
+/*
+ *  Copyright 2025-present the original author or authors.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *        https://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+@org.jspecify.annotations.NullMarked
+package org.springframework.integration.cloudevents;