Prefer Ollama configured model context length over model architecture context length

[MarekMalevic]

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Fixes calculation of Ollama model context length. Now prefers num_ctx model configuration over model architecture context length the same way Ollama does.

Closes #42340

Release Notes:

• Fix calculation of Ollama model context length

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[MarekMalevic]

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[probably-neb]

Thanks!