[agent builder] Update chat UI page and update all screenshots

[leemthompo]

closes https://github.com/elastic/docs-content-internal/issues/636

Search team issue: https://github.com/elastic/search-team/issues/12144

related PR: #4550

Summary

• Updated 5 chat UI screenshots (agent selector, chat input, history, main UI)
• Added new sections: model selector, more actions menu, reasoning/tool inspection panel
• Added :screenshot: directive to all images
• Minor content fixes (typo, link formatting)

Generative AI disclosure

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

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Claude Code, Opus 4.1

[github-actions]

Vale Linting Results

Summary: 2 suggestions found

💡 Suggestions (2)

File	Line	Rule	Message
explore-analyze/ai-features/agent-builder/agent-builder-agents.md	49	Elastic.WordChoice	Consider using 'copy' instead of 'Clone', unless the term is in the UI.
explore-analyze/ai-features/agent-builder/tools.md	216	Elastic.WordChoice	Consider using 'efficient' instead of 'easy', unless the term is in the UI.

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To use Vale locally or report issues, refer to Elastic style guide for Vale.

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/ai-features/agent-builder/agent-builder-agents.md
• explore-analyze/ai-features/agent-builder/chat.md
• explore-analyze/ai-features/agent-builder/tools.md
• explore-analyze/ai-features/agent-builder/tools/esql-tools.md

[charlotte-hoblik]

LGTM 🎉

[florent-leborgne]

🖼️