fix(istio-gateway): disable automatic passthrough values

General MR

Summary

This MR fixes an issue where all of the istio-gateway package's values were being copied into the upstream key, causing schema validation issues with the upstream chart. This was only noticeable when not using test-values as the test-values disable this functionality globally.

The version of istio-gateway first affected by this issue has not been included in a Big Bang release.

Relevant logs/screenshots

Before (without test values):

❯ kubectl get hr -n bigbang public-ingressgateway -o yaml | yq '.status.conditions[0]'
lastTransitionTime: "2025-09-19T16:16:58Z"
message: |-
  Helm install failed for release istio-gateway/public-ingressgateway with chart gateway@1.27.1-bb.0: values don't meet the specifications of the schema(s) in the following chart(s):
  upstream:
  - (root): Additional property networkPolicies is not allowed
  - (root): Additional property upstream is not allowed
  - (root): Additional property gateway is not allowed
observedGeneration: 1
reason: InstallFailed
status: "False"
type: Ready

After (without test values):

❯ kubectl get hr -n bigbang public-ingressgateway -o yaml | yq '.status.conditions[0]'
lastTransitionTime: "2025-09-19T16:20:28Z"
message: Helm install succeeded for release istio-gateway/public-ingressgateway.v1 with chart gateway@1.27.1-bb.0
observedGeneration: 1
reason: InstallSucceeded
status: "True"
type: Ready

Linked Issue

Closes #2880 (closed)

Upgrade Notices

N/A