Support Nebius Token Factory as a built-in model provider

[astrobot-houston]

Originally proposed by @amrrs in #281. Their description is reproduced verbatim below.

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adds Nebius Token Factory as a built-in OpenAI-compatible model provider for Flue.

This lets users configure agents with nebius/, authenticate with NEBIUS_API_KEY, and override Nebius transport settings through registerProvider('nebius', ...).

The PR also updates model/provider docs and adds focused runtime tests for model resolution and request authentication.

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Original implementation from #281 by @amrrs