[BUG]: React renderer shows developer `[Loading {id}...]` placeholders to end users

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

## Describe the Bug

The React renderer renders hard-coded developer placeholders directly into the surface tree, and there is no way to customize or disable them.

In `renderers/react/src/v0_9/A2uiSurface.tsx`, `DeferredChild` returns literal debug text when a component is not yet available:

```tsx
const componentModel = surface.componentsModel.get(id);

if (!componentModel) {
return

...
```

Because `A2uiSurface` renders ``, this placeholder sits on the only rendering path and cannot be bypassed. During streaming, when a referenced component has not been created yet, end users briefly see text such as `[Loading root...]`, and `Unknown component: ` (red) when a catalog implementation is missing. These are debug-style placeholders (inline `color`/`padding`) that are not appropriate for production UIs.

## Steps to Reproduce

1. Render a surface with `A2uiSurface`.
2. Stream component updates incrementally (e.g., send `createSurface`, then `updateComponents` in separate messages, or add a parent component before its children).
3. Observe the surface between updates.
4. See `[Loading root...]` (and `[Loading ...]` for not-yet-created children) rendered to the user.

## Expected Behavior

While a component is not yet available, the renderer should render nothing by default, or let the consumer provide a fallback. Debug text should not reach end users.

## Screenshots

## Environment Details

- **OS**: macOS
- **Browser/Platform**: Chrome
- **SDK/Package Name & Version**: @a2ui/react v0.10.1, @a2ui/web_core v0.10.3
- **Protocol Version**: v0.9
- **Agent Framework & LLM Model**: ADK

Issue は `renderers/react/src/v0_9/A2uiSurface.tsx` の `DeferredChild` コンポーネントにあります。まず、`componentModel` がどのように取得され、placeholder がどのようにレンダリングされるかを調べます。目的は、デバッグテキスト `[Loading {id}...]` の代わりに何も表示しないか、設定可能なフォールバックを表示するよう renderer を変更することです。surface のレンダリングロジックに既存のフォールバックパターンや props がないか確認します。説明されている streaming シナリオを再現して修正を検証します。

索引モデルが issue の本文から書いたものです。