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/src/__tests__/copilot-provider.spec.ts

@@ -328,7 +328,7 @@ const actions = [
     messages: [
       {
         role: 'user' as const,
-        content: '',
+        content: 'transcript the audio',
         attachments: [
           'https://cdn.affine.pro/copilot-test/MP9qDGuYgnY+ILoEAmHpp3h9Npuw2403EAYMEA.mp3',
         ],
@@ -350,7 +350,7 @@ const actions = [
     messages: [
       {
         role: 'user' as const,
-        content: '',
+        content: 'transcript the audio',
         attachments: [
           'https://cdn.affine.pro/copilot-test/2ed05eo1KvZ2tWB_BAjFo67EAPZZY-w4LylUAw.m4a',
         ],
@@ -372,7 +372,7 @@ const actions = [
     messages: [
       {
         role: 'user' as const,
-        content: '',
+        content: 'transcript the audio',
         attachments: [
           'https://cdn.affine.pro/copilot-test/nC9-e7P85PPI2rU29QWwf8slBNRMy92teLIIMw.opus',
         ],

packages/backend/server/src/__tests__/mocks/copilot.mock.ts

@@ -111,7 +111,7 @@ export class MockCopilotProvider extends OpenAIProvider {
       ],
     },
     {
-      id: 'gemini-2.5-pro-preview-03-25',
+      id: 'gemini-2.5-pro-preview-05-06',
       capabilities: [
         {
           input: [ModelInputType.Text, ModelInputType.Image],