Fix JAXB compilation warnings in geode-wan distributedTest (#7947)

Add javax.xml.bind:jaxb-api dependency to geode-wan's distributedTest
compile classpath to resolve compilation warnings about missing
XmlAccessType enum constant.

Problem:
The geode-wan module's distributedTest compilation was generating
warnings:
  warning: unknown enum constant XmlAccessType.FIELD
  reason: class file for jakarta.xml.bind.annotation.XmlAccessType
  not found

Root Cause:
The distributed test code references JAXB annotations (such as
@XmlAccessorType) through transitive dependencies from geode-core
and other modules, but the JAXB API was not explicitly declared
as a compile-time dependency for the distributedTest source set.
This caused the Java annotation processor to be unable to resolve
the XmlAccessType enum during compilation.

Solution:
Added 'javax.xml.bind:jaxb-api' to distributedTestCompileOnly
configuration in geode-wan/build.gradle. This ensures the JAXB
API is available during compilation of distributed test code,
allowing the annotation processor to properly resolve JAXB
annotations.

The compileOnly scope is appropriate here since:
- JAXB API is only needed at compile time for annotation processing
- Runtime implementation is provided by other modules' dependencies
- Keeps the test classpath minimal

Testing:
Verified with: ./gradlew :geode-wan:compileDistributedTestJava
Build completes successfully without JAXB warnings.

Related modules using similar pattern:
- geode-core: has jaxb-api in implementation scope
- geode-gfsh: has jaxb-api in implementation scope
- geode-connectors: has jaxb-api in implementation scope
- geode-web-api: has jaxb-api in implementation scope

Author: JinwooHwang

geode-wan/build.gradle

@@ -52,6 +52,7 @@ dependencies {
   distributedTestImplementation(project(':geode-dunit'))
   distributedTestImplementation(project(':geode-junit'))
 
+  distributedTestCompileOnly('javax.xml.bind:jaxb-api')
   distributedTestImplementation('mx4j:mx4j')
   distributedTestImplementation('org.awaitility:awaitility')
   distributedTestImplementation('junit:junit')