[DOCS]: a2uiClientCapabilities format in v0.9 documentation and schema

- [ ] I have searched the existing issues to make sure this documentation issue has not already been reported.

## Problem Page(s)

- `specification/v0_9_1/json/client_capabilities.json`
- https://a2ui.org/concepts/catalogs/#step-2-client-advertises-its-support-catalogs-required
- https://a2ui.org/concepts/actions/#advertising-capabilities

## Describe the Issue

I am validating `a2uiClientCapabilities` against the v0.9 schema, but the formats differ.

The schema and Actions documentation show:

```json
{
"v0.9": {
"supportedCatalogIds": ["..."]
}
}
```

The Catalogs documentation shows:

```json
{
"a2uiClientCapabilities": {
"supportedCatalogIds": ["..."]
}
}
```

Would love some clarity whether the `"v0.9"` wrapper is required.

## Proposed Changes

Update the schema and documentation examples to consistently show the canonical format and remove the trailing comma.

## Additional Context

This prevents clients from reliably validating the documented payload.

La issue indica tre posizioni: il file di schema in specification/v0_9_1/json/client_capabilities.json e due pagine di documentazione. Confronta gli esempi per vedere la discrepanza tra il wrapper 'v0.9' e la chiave 'a2uiClientCapabilities'. Aggiorna lo schema e la documentazione in modo che corrispondano al formato canonico, assicurandoti di rimuovere le virgole finali. Verifica le modifiche controllando la sintassi JSON e la coerenza tra i file.

Scritto dal modello di indicizzazione a partire dal testo della issue.