Add Nebius Token Factory provider

[amrrs]

Summary

Adds Nebius Token Factory as a first-class provider. Like Together and Fireworks, it serves many labs' models behind one key over an OpenAI-compatible API, so it slots into the existing reseller shape — no new client code, just a _compat descriptor plus curated matrix rows in Nebius' own model namespaces.

• providers/registry.py: one _compat descriptor (https://api.tokenfactory.nebius.com/v1, NEBIUS_API_KEY).
• providers/matrix.py: four curated nebius: rows (GLM-5.2, DeepSeek V4 Pro, Kimi K2.6, Qwen3.5 397B).
• GUI: the brand mark (vendored from the same MIT lobe-icons set the other logos come from), its slot in PROVIDER_ORDER, and the "where do I get a key" deep link.
• tests/test_providers.py: nebius folded into the two existing reseller assertions (matrix capabilities, descriptor ⇄ matrix lockstep).

25 insertions across 6 files. The endpoint help notes that the former AI Studio host (api.studio.nebius.com/v1) still resolves, for anyone with an older key.

Verification against the live API (2026-07-26)

Run through the real code path (build_provider_client("nebius", …)) with a live key:

• verify_provider_key → {'ok': True}; a bad key → {'ok': False, 'error': 'Invalid API key.'}
• Tool calling on all four curated ids, e.g. ToolCall(id='call_0eaf…', name='get_weather', arguments={'city': 'Paris'})
• Plain completions and streaming both fine; reasoning_content deltas surface as reasoning (the existing DeepSeek-style path), so thinking text is captured rather than mixed into the answer.
• Capabilities resolve exactly from the matrix: tools=True parallel_tool_calls=True streaming=True pdf=False — PDFs correctly fall back through pdf_support.py, like every other compat vendor.

Test suites

• pytest — 889 passed, 1 skipped
• npm run build (tsc + vite) — clean
• npm test (vitest) — 68 passed
• npm run e2e (Playwright) — 154 passed

Screenshots

Before, the provider gallery had no way to reach Nebius — a user had to know the "point the OpenAI slot at a different endpoint" trick. Now it is a card like any other:

Its setup form gets the prefilled endpoint, the key deep link, and the four curated models:

And Test → save flips the card to ✓ Connected:

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