Allow agent and client codebases to have a shared Catalog API source of truth e.g. catalog.json

From @zeroasterisk in https://github.com/a2ui-project/a2ui/issues/825:

> 3. No shared catalog schema
> Client and agent must independently define component names/properties. No way to auto-generate agent prompts from client catalog.
>
> Suggestion: A catalog.json schema file consumed by both sides.

Filing this issue specifically about this part.

Some ideas:
- Have developer programmatically define the catalog API in client code, then "export" it somehow to JSON document that gets pasted into the server codebase, perhaps through some NPM tooling

- Have a way to codegen the catalog implementation stubs from a JSON file, which is used by both server and client codebases.

在 client 和 agent 兩個程式碼庫中尋找現有的目錄定義，以了解目前的重複情況。調查 catalog API 目前是如何定義和使用的。確定一種產生或共用單一 schema (catalog.json) 的方法，讓雙方都能使用該 schema，也可以考慮使用程式碼產生工具。目標是擁有一個共用的單一事實來源，消除各自獨立的定義。

由索引模型根據 Issue 內容生成。