feat(core): add optionalModels field in AiPrompt and support the front-end modelId param (#12224)

Close [AI-116](https://linear.app/affine-design/issue/AI-116)

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

- **New Features**
  - Added support for specifying alternative AI models in chat prompts, enabling users to select from multiple available models.
  - Expanded AI model options with new additions: 'gpt-4.1', 'o3', and 'claude-3-5-sonnet-20241022'.

- **Enhancements**
  - Users can now optionally choose a specific AI model during chat interactions.
  - Prompts and chat sessions reflect and support selection of alternative models where applicable.

- **Bug Fixes**
  - Improved handling of prompt configuration defaults for better reliability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/session.ts

@@ -45,6 +45,10 @@ export class ChatSession implements AsyncDisposable {
     return this.state.prompt.model;
   }
 
+  get optionalModels() {
+    return this.state.prompt.optionalModels;
+  }
+
   get config() {
     const {
       sessionId,