When enabled, JMX metrics reporter throws exception on startup

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Fluss version

main (development)

### Please describe the bug 🐞

When enabled, the JMX reporters fails with following error:

```
2026-02-24 15:26:40,487 ERROR org.apache.fluss.metrics.reporter.ReporterSetup [] - Could not instantiate MetricReporter with name jmx. Metrics might not be exposed/reported.
java.lang.IllegalAccessError: superclass access check failed: class org.apache.fluss.metrics.jmx.JMXServer$JmxRegistry (in unnamed module @0x41294f8) cannot access class sun.rmi.registry.RegistryImpl (in module java.rmi) because module java.rmi does not export sun.rmi.registry to unnamed module @0x41294f8
at java.lang.ClassLoader.defineClass1(Native Method) ~[?:?]
at java.lang.ClassLoader.defineClass(Unknown Source) ~[?:?]
at java.security.SecureClassLoader.defineClass(Unknown Source) ~[?:?]
at java.net.URLClassLoader.defineClass(Unknown Source) ~[?:?]
at java.net.URLClassLoader$1.run(Unknown Source) ~[?:?]
at java.net.URLClassLoader$1.run(Unknown Source) ~[?:?]
at java.security.AccessController.doPrivileged(Unknown Source) ~[?:?]
at java.net.URLClassLoader.findClass(Unknown Source) ~[?:?]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:?]
at org.apache.fluss.classloading.ComponentClassLoader.loadClassFromComponentOnly(ComponentClassLoader.java:153) ~[fluss-server-0.8.0-incubating.jar:0.8.0-incubating]
at org.apache.fluss.classloading.ComponentClassLoader.loadClassFromOwnerFirst(ComponentClassLoader.java:175) ~[fluss-server-0.8.0-incubating.jar:0.8.0-incubating]
at org.apache.fluss.classloading.ComponentClassLoader.loadClass(ComponentClassLoader.java:110) ~[fluss-server-0.8.0-incubating.jar:0.8.0-incubating]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:?]
at org.apache.fluss.metrics.jmx.JMXServer.internalStart(JMXServer.java:94) ~[?:?]
at org.apache.fluss.metrics.jmx.JMXServer.start(JMXServer.java:62) ~[?:?]
at org.apache.fluss.metrics.jmx.JMXService.startJMXServerWithPortRanges(JMXService.java:90) ~[?:?]
at org.apache.fluss.metrics.jmx.JMXService.startInstance(JMXService.java:59) ~[?:?]
at org.apache.fluss.metrics.jmx.JMXReporter.(JMXReporter.java:75) ~[?:?]
at org.apache.fluss.metrics.jmx.JMXReporterPlugin.createMetricReporter(JMXReporterPlugin.java:32) ~[?:?]
at org.apache.fluss.metrics.jmx.JMXReporterPlugin.createMetricReporter(JMXReporterPlugin.java:25) ~[?:?]
at org.apache.fluss.metrics.reporter.ReporterSetup.setUpReporters(ReporterSetup.java:131) [fluss-server-0.8.0-incubating.jar:0.8.0-incubating]
at org.apache.fluss.metrics.reporter.ReporterSetup.fromConfiguration(ReporterSetup.java:59) [fluss-server-0.8.0-incubating.jar:0.8.0-incubating]
at org.apache.fluss.metrics.registry.MetricRegistry.create(MetricRegistry.java:72) [fluss-server-0.8.0-incubating.jar:0.8.0-incubating]
at org.apache.fluss.server.coordinator.CoordinatorServer.startServices(CoordinatorServer.java:164)
```

### Solution

Use `LocateRegistry.createRegistry` factory that uses JDK internals from inside the JDK module.

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Start at org.apache.fluss.metrics.jmx.JMXServer.internalStart, reported at JMXServer.java:94, and trace the startup path from JMXService.startJMXServerWithPortRanges. Review the JMX reporter setup and verify the behavior with JMX metrics enabled. Done means startup no longer throws the java.rmi module access error and the JMX reporter is available.

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