[API Nodes] use new API client in Luma and Minimax

[bigcat88]

1. Removed the old, unnecessary upload_file_to_comfyapi function. All nodes now use its new implementation.
2. Migrated Luma API nodes to the new API client and removed unused code (e.g., IO.Hidden.auth_token_comfy_org in non-API nodes).
3. Migrated Minimax nodes to the new API client. Also fixed backup_download_url handling: the node now makes two attempts to download from the China-based download_url; if both fail, it makes up to three attempts using the US-based backup_download_url.
4. Continuing to split Pydantic models into separate files. Rationale: the single backend-generated file is hard to update and sometimes uses hard-to-read names. Making a PR to backend to update some small value is hardly an option. Most nodes already use separate files for Pydantic models, and this change continues that direction. Additionally we may consider moving validation of inputs to Pydantic models with nice human readable error messages, once Pydantic reach amazing v3 on which they are working now.

Everything was tested, to ensure that nothing got broken:

[bigcat88]

+label: Core

[Kosinkadink]

looks good, merging