fix(helm): skip OCI repos in RepoUpdate

[immanuwell]

RepoUpdate passes all repos to helm repo update including OCI ones - that straight up fails since OCI registries dont use the index-based update flow. Pull already handles this correctly by special-casing oci:// URLs, RepoUpdate was just missing the same treatment. Filter them out, return early if nothing's left.

Also TestAddOCI was panicking (index out of range [0] with length 0) because assert is non-fatal and continued into listResult[0] after a failed check. Switched to require to fail fast.

Reproduce (panics before this fix):

go test ./pkg/helm/... -run TestAddOCI

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