chore: Add UT to ensure Arc charts are parseable

[MuhammadAliFleet]

Description of your changes

This PR adds a UT that ensures that the Arc member agent charts are parseable to avoid any misconfigured yaml

Fixes #

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Special notes for your reviewer

[Copilot]

Pull request overview

This PR adds unit tests for the Arc member agent Helm charts to ensure that all chart templates render to valid YAML. This is the first Helm chart test in the repository and validates that templates parse and render correctly with maximal configuration values to activate all conditional paths. The test helps prevent misconfigured YAML from being deployed.

• Adds comprehensive unit test for Helm template rendering validation
• Implements custom mock functions for Helm/Sprig template functions
• Tests all non-helper templates in the charts directory

[Ealianis]

This makes sense to add as we don't have E2E tests for when the helm chart is published. Our Internal E2Es join arc clusters after the extension has been created.

In the future, we can remove this once we have our own repo and E2E tests.