4. `parse_rsa_pem(pem_contents, display_path) -> Result<EncodingKey, GitHubError>`:
calls `validate_rsa_pem` first, then
`EncodingKey::from_rsa_pem(pem_contents.as_bytes())`, mapping
`jsonwebtoken::errors::Error` to `GitHubError::PrivateKeyLoadFailed`.