Reject private and loopback IPs in checkMetadataURL#21
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The URL check accepted any HTTPS host including loopback, link-local, and private IPs. A compromised registry could point download URLs at cloud metadata endpoints or internal services. Now resolves the hostname and rejects private, loopback, and link-local addresses.
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`checkMetadataURL` validated the scheme and presence of a host but accepted any IP, including `127.0.0.1`, `169.254.169.254`, and `::1`. A compromised registry response could point download URLs at cloud metadata endpoints or internal services.
Now resolves the hostname to an IP and rejects loopback, private, and link-local addresses. The redirect-following bypass noted in the existing code comments is left for the `Transport.DialContext` approach scoped in PR #15.