Add annotation for rendering of the RenderFragments in the case of serialization

## RenderFragment serialization does not support conditionally skipped fragments

### Problem

In the current implementation of `RenderFragment` serialization across render mode boundaries, we rely on the prerendering pipeline to execute all `RenderFragment` parameters so that we can capture their output via wrappers. This works only when the component unconditionally invokes every `RenderFragment` parameter during prerendering.

However, this is not always the case. A component may conditionally skip rendering a `RenderFragment`, which causes serialization to throw.

**Example:**

`MainComponent.razor`:
```razor

Condition is true

```

`SmallComponent.razor`:
```razor
@if (condition)
{
@ChildContent
}

@code {
[Parameter]
public RenderFragment ChildContent { get; set; } = default!;

[Parameter]
public bool condition { get; set; }
}
```

In this example, `ChildContent` is never invoked during prerendering because `condition` is `false`. The capture wrapper is set up but never executed, so `GetCapturedFrames()` throws an `InvalidOperationException` when serialization is attempted.

### Proposed Solution

Introduce an opt-in annotation (e.g., `[ExecuteChildContentForSerialization]`) that instructs the serializer to execute the `RenderFragment` directly via `RenderTreeBuilder` instead of throwing when the capture has no frames.

When this attribute is present on a `RenderFragment` parameter, the serializer would invoke the fragment explicitly to capture its output, even if the component's `BuildRenderTree` did not invoke it during prerendering.

Because this feature is opt-in rather than opt-out, we can safely assume that developers who apply the annotation understand the trade-off: the `RenderFragment` will be executed during prerendering regardless of any conditional logic in the component