[Pixtral] Add new model ; add vision

[Asterovim]

Summary

This PR adds support for new Mistral AI models, specifically adding configuration for Pixtral models (pixtral-large-latest and pixtral-large-2411) to the model runtime. These models support vision and agent-thought features with a large context window of 128K tokens.

https://mistral.ai/fr/news/pixtral-large/

Changes include:

• Added new models to _position.yaml
• Created configuration files for pixtral-large-2411 and pixtral-large-latest
• Models support vision and agent-thought capabilities
• Configured with standard parameter rules (temperature, top_p, max_tokens, safe_prompt, random_seed)
• Set pricing at $0.008/1K tokens for input and $0.024/1K tokens for output

Close #10896

Screenshots

Before:	After:
No Pixtral large models available in model selection	Pixtral large models available with vision capabilities

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[Asterovim]

Original issue : #10896

[crazywoola]

cc @laipz8200