[UI] Add coverage Addon Configuration Modal during cluster k8s version upgrade

[yonasberhe23]

Create an automated test to verify that the addon configuration confirmation modal functions correctly when editing a cluster's Kubernetes version.

Setup: Create a real RKE2 cluster with a specific Kubernetes version (e.g., v1.27.x)
Test Scenario:

1. Navigate to cluster edit page
2. Change Kubernetes version to trigger the addon confirmation modal
3. Verify modal appears
4. Validate cluster upgrade process completes without errors

Note: This test will need to be implemented in the Jenkins pipeline environment since it requires creating and managing real clusters.

rancher/dashboard#15183

[yonasberhe23]

The Addon Configuration Modal is difficult to automate due to the number of conditions required for it to display.

What I've attempted:

• Created a test that creates a cluster with an older Kubernetes version and upgrades it to trigger the modal
• Mocked catalog API calls to ensure different addon chart versions between old and new K8s versions
• Mocked user customizations (chartValues) in the cluster spec to simulate user-modified addon configs
• Intercepted and modified RKE2 releases API to ensure chart version differences
• Tried both real cluster creation (via API) and fake cluster mocking approaches
• Added timing waits to ensure initAddons() completes and versionInfo is populated before processing chartValues

Why it didn't work:

• The modal requires all of the following conditions to be met:
• Cluster must be in edit mode
• Kubernetes version must change
• Addon version must change between old and new K8s versions
• User must have customized addon values that conflict with the new addon version's defaults

The conditions make it difficult to reliably reproduce in an automated test.

Decision: Repurposing this ticket to add a Kubernetes version upgrade test instead, which verifies the upgrade functionality without requiring the modal to appear.