fix(helm): support OCI references in HTTP repository indexes

[abhinavgautam01]

Fixes #320

Summary

Helm HTTP repositories can include oci:// chart references in their index.yaml. The proxy previously rejected these references, causing the entire index request to fail with a 502 response.

Changes

• Accept OCI references alongside HTTP(S) chart URLs.
• Rewrite references matching configured OCI registries through the existing OCI proxy routes, preferring named registries.
• Preserve unmatched references and their repository paths, tags, and digests.
• Keep OCI references out of the HTTP chart download path.
• Preserve HTTP chart caching, digest verification, and cooldown filtering.
• Document registry matching and root-URL requirements.

Tests

Added coverage for mixed HTTP/OCI indexes, reference rewriting, invalid references, cooldown filtering, HTTP chart caching, OCI manifest and blob routing and server configuration wiring.

Local validation passed:

• Formatting and git diff --check
• go tool golangci-lint run ./... — 0 issues
• go vet ./...
• go build ./...
• go test -race ./...