RunnerSet has dynamic name

Checks

• I've already read https://github.com/actions-runner-controller/actions-runner-controller/blob/master/TROUBLESHOOTING.md and I'm sure my issue is not covered in the troubleshooting guide.
• I'm not using a custom entrypoint in my runner image

Controller Version

latest

Helm Chart Version

0.21.0

CertManager Version

1.8

Deployment Method

Helm

cert-manager installation

yes

Checks

• This isn't a question or user support case (For Q&A and community support, go to Discussions. It might also be a good idea to contract with any of contributors and maintainers if your business is so critical and therefore you need priority support
• I've read releasenotes before submitting this issue and I'm sure it's not due to any recently-introduced backward-incompatible changes
• My actions-runner-controller version (v0.x.y) does support the feature
• I've already upgraded ARC (including the CRDs, see charts/actions-runner-controller/docs/UPGRADING.md for details) to the latest and it didn't fix the issue
• I've migrated to the workflow job webhook event (if you using webhook driven scaling)

Resource Definitions

apiVersion: actions.summerwind.dev/v1alpha1
kind: RunnerSet
metadata:
  name: default-runner
spec:
  ephemeral: true
  organization: Test
  replicas: 1
    spec:
      containers:
        - env:
            - name: STARTUP_DELAY_IN_SECONDS
              value: '2'
            - name: DISABLE_WAIT_FOR_DOCKER
              value: 'false'
            - name: DISABLE_RUNNER_UPDATE
              value: 'true'
          name: runner
        - env:
            - name: POD_NAME
              valueFrom:
                fieldRef:
                  apiVersion: v1
                  fieldPath: metadata.name
          name: docker
          volumeMounts:
            - mountPath: /var/lib/docker
              name: docker
              subPathExpr: $(POD_NAME)
        volumes:
        - hostPath:
            path: /mnt/cache/docker
            type: DirectoryOrCreate
          name: docker

To Reproduce

create RunnerSet resource

Describe the bug

stateful set created with dynamic string in the name
"default-runner-kgd9h-0"

Describe the expected behavior

statefullset doesn't have dynamic string in the name:
"default-runner-0"

Whole Controller Logs

none

Whole Runner Pod Logs

none

Additional Context

No response

Start by creating the provided RunnerSet manifest and inspecting the generated StatefulSet name. Trace the controller entry point that derives StatefulSet names; done means the generated name matches the expected stable form without the dynamic segment, with regression coverage if existing tests expose this behavior.