Improve error message when pulling a nonexistent catalog

[jchangx]

Summary

• Detect MANIFEST_UNKNOWN and NAME_UNKNOWN OCI registry errors during catalog pull
• Return a clear message instead of the raw transport error

Before:

❯ docker mcp catalog pull mcp/unknown-catalog
failed to read OCI catalog: failed to fetch image/artifact mcp/unknown-catalog: GET https://index.docker.io/v2/mcp/unknown-catalog/manifests/latest: MANIFEST_UNKNOWN: manifest unknown; unknown tag=latest

After:

❯ docker mcp catalog pull mcp/unknown-catalog
catalog not found: mcp/unknown-catalog

[chatgpt-codex-connector]

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[xenoscopic]

Clean, focused fix. The isNotFoundError helper correctly uses errors.As to unwrap through the OCI transport error chain and checks both MANIFEST_UNKNOWN and NAME_UNKNOWN codes. Test coverage is thorough including wrapped errors, non-matching transport errors, empty diagnostics, and mixed diagnostic lists. All CI green. LGTM.