feat(core): add google vertex ai (#12423)

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

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

- **New Features**
  - Added new provider configurations `geminiVertex` and `anthropicVertex` for Google Vertex AI in backend schema, provider classes, and admin config.
  - Introduced `GeminiVertexProvider` and `AnthropicVertexProvider` classes supporting Vertex AI models with specific capabilities.
  - Expanded model options for transcription prompts with newer Gemini models.
  - Re-exported provider modules to include Vertex AI variants.

- **Improvements**
  - Extended provider architecture to support separate Vertex AI configurations and models.
  - Updated test setup to replace deprecated provider references with new Vertex variants.
  - Consolidated environment variables for server testing with a single `SERVER_CONFIG`.

- **Bug Fixes**
  - Updated mock models and import references in tests to align with new provider classes.

- **Chores**
  - Added `@ai-sdk/google-vertex` dependency for Vertex AI support.
  - Updated dependency list to include `@ai-sdk/google-vertex`.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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

@@ -111,7 +111,7 @@ export class MockCopilotProvider extends OpenAIProvider {
       ],
     },
     {
-      id: 'gemini-2.5-pro-preview-05-06',
+      id: 'gemini-2.5-flash-preview-05-20',
       capabilities: [
         {
           input: [ModelInputType.Text, ModelInputType.Image],