fix: handle exceptions during Nacos service shutdown to prevent NPE propagation

[daguimu]

Describe what this PR does / why we need it

During application shutdown, NacosServiceManager.nacosServiceShutDown() calls namingService.shutDown() which internally invokes NotifyCenter.deregisterSubscriber(). If the JVM shutdown hooks have already destroyed NotifyCenter.INSTANCE, this throws a NullPointerException:

java.lang.NullPointerException: Cannot read field "sharePublisher" because "com.alibaba.nacos.common.notify.NotifyCenter.INSTANCE" is null
  at com.alibaba.nacos.common.notify.NotifyCenter.deregisterSubscriber(NotifyCenter.java:242)
  at com.alibaba.nacos.client.naming.remote.gprc.NamingGrpcClientProxy.shutdown(...)
  at com.alibaba.cloud.nacos.NacosServiceManager.nacosServiceShutDown(...)
  at com.alibaba.cloud.nacos.registry.NacosServiceRegistry.close(...)

This NPE escapes NacosServiceRegistry.close() (which only catches NacosException) and propagates to NacosGracefulShutdownDelegate, causing an alarming ERROR log during every application shutdown.

Additionally, when namingService.shutDown() throws, namingMaintainService.shutDown() never gets a chance to execute.

Does this pull request fix one issue?

Fixes #4081
Fixes #4247

Describe how you did it

Wrapped each shutDown() call in NacosServiceManager.nacosServiceShutDown() with independent try-catch blocks:

• NacosException is caught and logged at ERROR level (real failures)
• Other exceptions (including the NPE race condition) are caught and logged at WARN level (expected during shutdown ordering)
• Service references are always nulled out after shutdown attempt
• Both namingService and namingMaintainService always get their shutdown attempts, even if one fails

Describe how to verify it

1. Start a Spring Cloud Alibaba application with Nacos discovery
2. Stop the application
3. Verify no ERROR log from NacosGracefulShutdownDelegate during shutdown
4. Run NacosServiceManagerTests — all 6 tests pass

Tests Added

Change Point	Test
namingService.shutDown() NPE caught at WARN	testNamingServiceNpeShutdownShouldNotPropagate()
namingMaintainService.shutDown() NPE caught at WARN	testNamingMaintainServiceNpeShutdownShouldNotPropagate()
Both services get shutdown attempt when first fails	testBothServicesFailShouldStillCleanUp()
NacosException caught and logged at ERROR	testNacosExceptionDuringShutdownShouldNotPropagate()
Normal shutdown still works	testNormalShutdownShouldSucceed()
Null services handled safely	testNullServicesShouldNotFail()

Special notes for reviews

• The method signature throws NacosException is preserved for backward compatibility, though exceptions are now handled internally
• NacosServiceRegistry.close() catch block becomes dead code but is left unchanged to minimize diff
• The existing NacosGracefulShutdownDelegateTests (4 tests) continue to pass

[CLAassistant]

All committers have signed the CLA.