[Test] Complete TODO assertion in NamingUtilsTest for valid clusterName case

[Sunwenzhi58]

Background

In NamingUtilsTest, method testBatchCheckInstanceIsLegal contains a TODO comment:

• file: api/src/test/java/com/alibaba/nacos/api/naming/utils/NamingUtilsTest.java
• TODO: valid clusterName

This test path currently executes but has weak assertion style and can be improved for readability and maintainability.

Proposal

Refine the TODO section to use clearer assertion style and ensure the valid clusterName path is explicitly validated.

Scope:

• test file only
• no production code change

Why this helps

• Improves test readability for new contributors
• Makes expected behavior clearer in batch validation scenario
• Zero runtime risk since this is test-only

Plan

• Update TODO section in testBatchCheckInstanceIsLegal
• Keep behavior unchanged
• Run module test: mvn -pl api -Dtest=NamingUtilsTest test