Where does a reported client data model ride in MCP _meta?

**Documents:** `docs/public/guides/a2ui_over_mcp.md`,
`specification/v0_9_1/json/server_to_client.json` (v1.0:
`specification/v1_0/json/agent_to_renderer.json`)

A server asks for the client's data model with `sendDataModel` on `createSurface` or
`updateDataModel`. Over A2A the client answers with `a2uiClientDataModel` in transport metadata,
and the A2A extension names that placement.

We could not find the equivalent for MCP. The guide defines
`capabilities.a2ui.clientCapabilities` at initialize and `_meta.a2ui.clientCapabilities` per call,
and says nothing further about the reported data model. Did we miss it somewhere?

If it is not defined yet, which placement do you intend?

- `_meta["a2ui"]["clientDataModel"]`, gathering A2UI metadata under `a2ui` and dropping the prefix
from the inner names, the way the guide already does with `clientCapabilities`.
- `_meta["a2uiClientDataModel"]`, keeping the spec name at the root of `_meta`.

We had one implementation of each, and nothing surfaced. MCP `_meta` is a free form object, so the
reader that looked in the wrong place saw no key and no error.

Either choice works for us. Could the guide name one?