Function resolve_digest 

pub async fn resolve_digest(
    client: &Client,
    registry: &str,
    repository: &str,
    tag: &str,
) -> Result<String, GhcrError>

Resolve the immutable manifest digest for repository:tag against registry.

The auth strategy is chosen from the host by auth_for: a public GHCR repository authenticates with an anonymous bearer token from the registry token endpoint, while a Google Artifact Registry / GCR host authenticates with a GCP OAuth access token.

The GET reads the Docker-Content-Digest response header — the registry computes it over the manifest bytes, so it is the canonical content address.

Errors

Returns GhcrError if the bearer token can’t be obtained, the manifest request fails, or the response carries no digest header.