[Bug]: ag-ui-a2ui-toolkit BASIC_CATALOG_ID does not match the A2UI v0.9 spec canonical catalog $id (surfaces rejected by @a2ui/react)

## Summary

`ag_ui_a2ui_toolkit.BASIC_CATALOG_ID` (v0.0.4) is `https://a2ui.org/specification/v0_9/basic_catalog.json`, which does **not** match the canonical A2UI v0.9 basic catalog `\$id` used by the reference renderer `@a2ui/react`. Surfaces emitted with the toolkit's constant are rejected at render time with `A2uiStateError: Catalog not found`.

## Evidence

The A2UI v0.9 basic catalog published at the spec host declares its own `\$id`:

```
\$ curl -s https://a2ui.org/specification/v0_9/catalogs/basic/catalog.json | head -5
{
"\$schema": "https://json-schema.org/draft/2020-12/schema",
"\$id": "https://a2ui.org/specification/v0_9/catalogs/basic/catalog.json",
"title": "A2UI Basic Catalog",
...
}
```

The toolkit's URL does not resolve:

```
\$ curl -s -o /dev/null -w "%{http_code}\n" https://a2ui.org/specification/v0_9/basic_catalog.json
404
```

The reference renderer registers the catalog under the canonical spec path:

```js
// @a2ui/react/v0_9
basicCatalog.id === "https://a2ui.org/specification/v0_9/catalogs/basic/catalog.json"
```

## Impact

A backend using `create_surface(surface_id, BASIC_CATALOG_ID)` from `ag_ui_a2ui_toolkit` produces a surface whose `catalogId` is `.../basic_catalog.json`. When the `@a2ui/react` `MessageProcessor` (initialized with `basicCatalog`) processes that surface, it throws:

```
A2uiStateError: Catalog not found: https://a2ui.org/specification/v0_9/basic_catalog.json
```

because no catalog is registered under that ID. The surface never renders.

## Expected

`BASIC_CATALOG_ID` should equal the canonical spec `\$id`:

```python
BASIC_CATALOG_ID = "https://a2ui.org/specification/v0_9/catalogs/basic/catalog.json"
```

## Environment

- `ag-ui-a2ui-toolkit` 0.0.4 (PyPI)
- `@a2ui/react` (renderer, a2ui.org reference implementation)

## Workaround

Consumers currently hardcode the canonical spec URL instead of importing the constant:

```python
BASIC_CATALOG_ID = "https://a2ui.org/specification/v0_9/catalogs/basic/catalog.json"
```