feat(core): add gemini callout syntax highlighting (#12413)

Close [AI-125](https://linear.app/affine-design/issue/AI-125)

What Changed?
- Add `gemini-2.5-flash-preview-04-17` model
- Add `thinkingConfig` provider options
- Add callout syntax highlighting

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

- **New Features**
  - Added support for the "Gemini 2.5 Flash" model and updated the "Gemini 2.5 Pro" model to a newer version.
  - Enhanced streaming responses to better format reasoning outputs and provide clearer callouts in AI-generated content.

- **Bug Fixes**
  - Improved audio transcription prompts in test cases for more accurate and explicit testing.

- **Documentation**
  - Expanded citation instructions for AI chat responses, including examples for multiple citations.

- **Chores**
  - Updated the "@ai-sdk/google" dependency to a newer version.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/package.json

@@ -29,7 +29,7 @@
     "@affine/reader": "workspace:*",
     "@affine/server-native": "workspace:*",
     "@ai-sdk/anthropic": "^1.2.10",
-    "@ai-sdk/google": "^1.2.10",
+    "@ai-sdk/google": "^1.2.18",
     "@ai-sdk/openai": "^1.3.21",
     "@ai-sdk/perplexity": "^1.1.6",
     "@apollo/server": "^4.11.3",