Update the existing Nebius provider for Token Factory

Problem

aisuite's existing Nebius provider still defaults to the retired Nebius AI Studio endpoint (https://api.studio.nebius.ai/v1) and its guide/key error links to the former Studio product. The current Token Factory quickstart uses https://api.tokenfactory.nebius.com/v1.

The existing provider is otherwise the correct ownership point; no second provider or Responses integration is needed.

Prepared repair

A narrow tested branch is available for review:

https://github.com/demianarc/aisuite/compare/andrewyng:main...demianarc:aisuite:codex/nebius-token-factory

It:

• updates the existing provider's base URL and branding;
• points key setup and documentation to Token Factory;
• changes the guide to an active model ID from the public catalog;
• adds deterministic tests for the exact constructor URL/auth configuration and missing-key guidance;
• retains the existing Chat Completions request/tool behavior and provider identity.

Verification

$ .venv/bin/pytest tests/providers/test_nebius_provider.py tests/client/test_client.py -q
42 passed in 0.34s

$ .venv/bin/black --check aisuite/providers/nebius_provider.py tests/providers/test_nebius_provider.py
All done! 2 files would be left unchanged.

$ git diff --check
passed

Sources

• Token Factory quickstart: https://docs.tokenfactory.nebius.com/quickstart
• Public model catalog: https://tokenfactory.nebius.com/model-catalog.md
• Token Factory documentation: https://docs.tokenfactory.nebius.com/