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/__tests__/copilot-provider.spec.ts

@@ -333,7 +333,7 @@ The term **“CRDT”** was first introduced by Marc Shapiro, Nuno Preguiça, Ca
     messages: [
       {
         role: 'user' as const,
-        content: 'what is ssot',
+        content: 'what is AFFiNE AI?',
         params: {
           files: [
             {