fix: update Nebius provider for Token Factory

[demianarc]

Description

Fixes #13128.

Continue's existing Nebius provider still used the retired Nebius AI Studio host in both the core LLM and the OpenAI adapter. It also rewrote model names through a static alias table that had drifted from the provider catalog; notably, llama3.1-8b resolved to the 70B model.

This change:

• switches both defaults to the documented https://api.tokenfactory.nebius.com/v1/ endpoint while preserving custom apiBase overrides
• removes the stale alias rewrites so configured and dynamically fetched catalog IDs pass through unchanged
• replaces the old preset list with a current starter plus Continue's existing authenticated live-catalog refresh
• updates the provider UI, config schema, and docs to Nebius Token Factory naming and current links
• removes the unverified pricing/trial copy and makes model-dependent tool support explicit
• adds mocked regressions for the default endpoint, custom endpoint override, streamed Chat Completions, tool payloads, and model-ID pass-through

Chat Completions remains the default API surface; this PR does not enable or claim stateful Responses API behavior.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not applicable: this repairs provider routing, catalog configuration, and copy without changing the form layout.

Tests

• cd core && npx vitest run llm/llms/OpenAI-compatible.vitest.ts — 212 passed
• cd core && npx vitest run llm/llms/OpenAI-compatible.vitest.ts -t nebius — 8 passed
• cd packages/openai-adapters && npx vitest run src/test/main.test.ts — 17 passed
• cd packages/openai-adapters && npm run build
• cd core && npm run tsc:check
• cd gui && npx tsc --project tsconfig.json --noEmit
• focused Prettier check and git diff --check

The API behavior tests are mocked and do not require or expose a live Nebius API key.

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