[FLINK-37893][k8s] Make REST service port name configurable

[1fanwang]

What is the purpose of the change

The Kubernetes REST service exposed by the JobManager hardcodes its port name to "rest" (Constants.REST_PORT_NAME). Operators in environments that enforce port-naming policies — for example, service meshes that route by port name, or organizations with strict port-name conventions — cannot align Flink's rest service with those policies.

This change adds a new config option kubernetes.rest-service.port-name (string, default "rest") that controls the port name on the REST Service. The default preserves existing behavior, making this a purely additive change.

Brief change log

• New config option kubernetes.rest-service.port-name in KubernetesConfigOptions (default "rest")
• New getter KubernetesJobManagerParameters#getRestServicePortName()
• ServiceType#buildUpExternalRestService uses the configured name instead of Constants.REST_PORT_NAME
• ServiceType#getRestPortFromExternalService no longer filters ports by name. Flink always builds the rest Service with a single port, so the lookup can simply return that port; this lets the reader work for any configured name without plumbing it through the abstract getRestEndpoint signature on ServiceType (which would change a public surface and require updating all subclasses).
• KubernetesResourceManagerDriver#updateKubernetesServiceTargetPortIfNecessary reads the configured port name from flinkConfig when updating the rest service's target port in host-network mode

Verifying this change

This change added tests and can be verified as follows:

• ExternalServiceDecoratorTest#testDefaultRestServicePortName — verifies the default port name "rest" is used when the option is unset
• ExternalServiceDecoratorTest#testCustomRestServicePortName — verifies a custom port name ("flink-rest") propagates to the built Service
• ServiceTypeTest#testGetRestPortFromExternalServiceWithDefaultName — verifies the reader works for a Service with the default port name (back-compat)
• ServiceTypeTest#testGetRestPortFromExternalServiceWithCustomName — verifies the reader works for a Service with a custom port name
• ServiceTypeTest#testGetRestPortFromExternalServiceFailsWhenNoPorts — verifies the error path when the Service has no ports
• All 106 existing tests in the kubeclient/services/decorators packages continue to pass (mvn -pl flink-kubernetes -Dtest='*KubeClient*,*ServiceType*,*ServiceDecorator*,*JobManagerDecorator*,KubernetesResourceManagerDriverTest,KubernetesUtilsTest' test)

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): yes — KubernetesConfigOptions is @PublicEvolving. Adds a new option (additive).
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: yes — affects native Kubernetes JobManager Service builder and KubernetesResourceManagerDriver. No behavior change at default.
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? config option withDescription (auto-generated into the Flink config docs)

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Generated-by: Claude Code (claude-opus-4-7)

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CI report:

• 43d6396 Azure: FAILURE

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