Add new server endpoint for installing OCI charts without Helm repository

[sowmya-sl]

Add dedicated endpoint and handler for installing Helm charts from OCI
registries. This provides a simplified installation flow for OCI charts
that doesn't require HelmChartRepository lookup.

Changes:

• Add InstallOCIChart function in actions for OCI-only installations
• Add HandleInstallOCIChart handler with OCI URL validation
• Register /api/helm/oci-chart endpoint in server routes
• Add tests for both the methods

HELM-598

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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This pull-request has been approved by: sowmya-sl
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