fix: better oci handling

[bosbaber]

This pull request improves the handling of Helm chart repository URLs by ensuring that scheme-less OCI registry URLs are normalized to use the oci:// scheme, matching the expectations of the Helm CLI and ArgoCD ApplicationSets. It introduces a normalization function, updates the chart rendering logic to use it, and adds comprehensive unit tests to verify the new behavior.

Repository URL normalization:

• Added a normalizeRepoURL function in utils.go to canonicalize ApplicationSet repoURL values, ensuring that scheme-less OCI registry URLs are prepended with oci://, while existing http://, https://, and oci:// schemes are preserved. This prevents Helm CLI errors and aligns with ArgoCD's behavior.
• Updated the chart rendering logic in engine_chart_rendering.go to normalize chart.RepoURL before invoking Helm, ensuring consistent handling of repository URLs.

Testing improvements:

• Added a new test TestNormalizeRepoURL in utils_test.go to cover various input cases for normalizeRepoURL, including empty strings, whitespace, explicit schemes, and scheme-less URLs.
• Introduced a test TestRenderOCIRepoNoScheme in engine_chart_rendering_test.go to verify that scheme-less OCI URLs are normalized and used correctly in Helm commands during chart rendering.