Fix rad upgrade kubernetes to preserve existing Helm release values

[Copilot]

Description

rad upgrade kubernetes was silently resetting non-default Helm values (e.g., global.azureWorkloadIdentity.enabled, database.enabled, custom registries) to chart defaults. Values set during install via --set were lost unless explicitly repeated on every upgrade.

Changes

Core fix (pkg/cli/helm/helmclient.go):

• Set upgradeClient.ReuseValues = true in RunHelmUpgrade()
• Values now merge: existing release values → chart defaults → CLI --set overrides

Test coverage (pkg/cli/helm/helmclient_test.go):

• Integration test validates value preservation using Helm's in-memory storage
• Tests specific scenario from issue: existing azureWorkloadIdentity.enabled=true preserved while applying new imageTag

Documentation (pkg/cli/cmd/upgrade/kubernetes/kubernetes.go):

• Updated command help to clarify value preservation behavior

Example

Before:

rad install kubernetes --set global.azureWorkloadIdentity.enabled=true
rad upgrade kubernetes
# azureWorkloadIdentity silently disabled ❌

After:

rad install kubernetes --set global.azureWorkloadIdentity.enabled=true
rad upgrade kubernetes
# azureWorkloadIdentity preserved ✅

rad upgrade kubernetes --set global.imageTag=0.48
# azureWorkloadIdentity still enabled, imageTag updated ✅

Type of change

• This pull request fixes a bug in Radius and has an approved issue (issue link required).

Contributor checklist

Please verify that the PR meets the following requirements, where applicable:

• An overview of proposed schema changes is included in a linked GitHub issue.
  • Yes
  • Not applicable
• A design document PR is created in the design-notes repository, if new APIs are being introduced.
  • Yes
  • Not applicable
• The design document has been reviewed and approved by Radius maintainers/approvers.
  • Yes
  • Not applicable
• A PR for the samples repository is created, if existing samples are affected by the changes in this PR.
  • Yes
  • Not applicable
• A PR for the documentation repository is created, if the changes in this PR affect the documentation or any user facing updates are made.
  • Yes
  • Not applicable
• A PR for the recipes repository is created, if existing recipes are affected by the changes in this PR.
  • Yes
  • Not applicable

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