Helm: Adds istio as a provider-scoped value for the inferencepool Chart

[danehans]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Adds istio as a provider-scoped value since Istio is considered a provider. The current top-level istio value is preserved for backwards compatibility while the new scope takes precedence. The top-scoped istio value should be removed in the v.1.3 release.

Which issue(s) this PR fixes:

Fixes #1829

Does this PR introduce a user-facing change?:

Helm: Adds `istio` as a `provider`-scoped value of the inferencepool chart. 

[netlify]

✅ Deploy Preview for gateway-api-inference-extension ready!

Name	Link
🔨 Latest commit	42b9752
🔍 Latest deploy log	https://app.netlify.com/projects/gateway-api-inference-extension/deploys/690e46d5850d490008585888
😎 Deploy Preview	https://deploy-preview-1831--gateway-api-inference-extension.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: danehans

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [danehans]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[danehans]

Tested using different combinations of istio values, and the destrule is managed properly:

$ k get destinationrules.networking.istio.io -A -o yaml
apiVersion: v1
items:
- apiVersion: networking.istio.io/v1
  kind: DestinationRule
  metadata:
    annotations:
      meta.helm.sh/release-name: vllm-llama3-8b-instruct
      meta.helm.sh/release-namespace: default
    creationTimestamp: "2025-11-07T19:13:23Z"
    generation: 1
    labels:
      app.kubernetes.io/managed-by: Helm
    name: vllm-llama3-8b-instruct-epp
    namespace: default
    resourceVersion: "11470"
    uid: 1d443e5e-82a5-4b48-9aa4-0b0a1f7faa32
  spec:
    host: vllm-llama3-8b-instruct-epp.default.svc.cluster.local
    trafficPolicy:
      tls:
        insecureSkipVerify: true
        mode: SIMPLE
kind: List
metadata:
  resourceVersion: ""