Use the selected chat model for the Ruby design agent

[Hashim1999164]

Summary

• The design chat agent always asked vscode.lm.selectChatModels for Copilot GPT4o, so custom chat models left model undefined and sendRequest crashed
• It now uses the model already selected for the chat, falls back to Copilot GPT4o if needed, and shows a message instead of throwing when no model is available

Closes #4186

Tests

• npx tsc covering the vscode extension