Fix and improve Nebius provider. Add LLMs and descriptions

[vadjs]

Description

• Added description for the Nebius provider (sorry for not doing so earlier)
• Fixed Provider name in Nebius provider
• Added model name mapping in Nebius provider to have more consistent model names
• Added new model variants

Checklist

• The relevant docs, if any, have been updated or created

Testing

Please add the new NEBIUS_API_KEY environment variable for the tests.

To do it:

• Go to https://studio.nebius.ai/settings/api-keys
• You should automatically get $100 which should be enough for a lifetime testing.
• Create API key

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[sestinj]

looks great, thanks!