[Feature Request] Configure Global Available Models

## Describe the solution you'd like

- Add configuration option in cdk.json or parameter.ts to globally control which models appear in dropdown menus across all chats
Implement this as a new parameter like globalAvailableModels that accepts an array of model IDs
- Or, add document where need to be modified

## Why the solution needed

#798 #832

## Implementation feasibility

Are you willing to collaborate with us to discuss the solution, decide on the approach, and assist with the implementation?

- [ ] Yes, I am able to implement the feature and create a pull request.
- [ ] No, I am unable to implement the feature, but I am open to discussing the solution.

Start by reading cdk.json and parameter.ts to locate existing configuration parameters, then trace how model IDs populate the chat dropdowns across the application. Done means a globalAvailableModels array can be configured and consistently controls the available models in every chat, with the relevant tests or documentation updated.

Written by the indexing model from the issue text.