[Obs AI] Update LLM performance matrix with new GPT model ratings

[viduni94]

Closes https://github.com/elastic/obs-ai-team/issues/477
Closes https://github.com/elastic/obs-ai-team/issues/476

Summary

This PR updates the Obs AI Assistant LLM performance matrix with 2 new GPT models

• GPT 4.1 Mini
• GPT 5.2

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

[github-actions]

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[github-actions]

🔍 Preview links for changed docs

• solutions/observability/llm-performance-matrix.md

[florent-leborgne]

LGTM - @viduni94 it may be good to list newer ones first if the list keeps expanding

[viduni94]

LGTM - @viduni94 it may be good to list newer ones first if the list keeps expanding

Thanks for the feedback @florent-leborgne
At the moment, the table follows alphabetical order. Should I update it to be newest models first?