Python a2ui_core: Provide a tool to generate model classes for any custom catalogs

https://github.com/a2ui-project/a2ui/pull/1575 introduces a [script](https://github.com/a2ui-project/a2ui/blob/main/agent_sdks/python/a2ui_core/scripts/generate_schemas.py) that generates model class based on the A2UI spec.

It would be good if we can provide a tool to generate model classes for any custom catalogs. See context in https://github.com/a2ui-project/a2ui/pull/1575#discussion_r3378063954.

Examine the existing script at agent_sdks/python/a2ui_core/scripts/generate_schemas.py to understand how it works with the A2UI spec. Review the linked PR discussion for context on custom catalogs. The task involves extending or creating a new tool that can accept a custom catalog definition and generate corresponding Python model classes. Start by understanding the structure of the spec and the generated classes, then design the tool's input and output.

索引モデルが issue の本文から書いたものです。