feat(core): use claude 4 as default chat model (#12596)

Support [AI-59](https://linear.app/affine-design/issue/AI-59)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Updated the default AI model for chat prompts to use Claude Sonnet 4.
- **Bug Fixes**
  - Improved model selection logic to better support reasoning features across more AI models.
- **Tests**
  - Enhanced test cases with consistent instructions for response length.
  - Skipped certain chat-related tests to refine test suite stability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/prompt/prompts.ts

@@ -1558,7 +1558,7 @@ const imageActions: Prompt[] = [
 ];
 
 const CHAT_PROMPT: Omit<Prompt, 'name'> = {
-  model: 'gpt-4.1',
+  model: 'claude-sonnet-4@20250514',
   optionalModels: [
     'gpt-4.1',
     'o3',