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 两个代码库中查找现有的目录定义，以了解当前的重复情况。调查 catalog API 当前是如何定义和使用的。确定一种生成或共享单一 schema (catalog.json) 的方法，使双方都能使用该 schema，可以考虑使用代码生成工具。目标是拥有一个共享的单一事实来源，消除各自独立的定义。

由索引模型根据 Issue 内容生成。