feat(services/hf): add download_mode option and require explicit repo_type

[kszucs]

Summary

• Add download_mode config option (xet | http) to the HuggingFace service. In xet mode (default) the resolve endpoint is called with Accept: application/vnd.xet-fileinfo+json and files are streamed via the XET SDK. In http mode the server's 302 redirect is followed and bytes are streamed directly without any XET SDK involvement.
• Move HfRepoType from uri.rs to config.rs alongside the new HfDownloadMode enum.
• Make repo_type explicit — HfRepoType no longer implements Default, HfConfig.repo_type is Option<HfRepoType>, and both from_uri() and build() return ConfigInvalid if it is not set.
• Consolidate path_resolve / path_resolve_http into a single resolve(path, range, mode) method on HfCore.
• Refactor parse_error to take (parts, body) directly instead of a full Response<Buffer>.