docs: add FuturMix as OpenAI-compatible provider

[FuturMix]

Summary

• Add FuturMix as a model provider option in the models tutorial (python/docs/src/user-guide/agentchat-user-guide/tutorial/models.ipynb)
• FuturMix is an enterprise AI agent platform providing access to 22+ models (OpenAI, Anthropic, Google)
• Section follows the same pattern as existing Gemini and Llama API entries, using OpenAIChatCompletionClient with a custom base_url
• Includes examples for both OpenAI models and cross-provider access (Claude, Gemini) with model_info configuration

Changes

Added three cells to models.ipynb after the Llama API section:

1. Markdown intro: Brief description of FuturMix with link
2. Code cell: Basic usage example with OpenAIChatCompletionClient
3. Markdown note: Cross-provider examples (Claude, Gemini) with model_info for non-OpenAI models

Test plan

• Verify the notebook renders correctly in the docs site
• Verify code examples follow existing patterns (consistent with Gemini/Llama API sections)
• Verify no existing content was modified

🤖 Generated with Claude Code

[FuturMix]

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