Support union-typed templated component parameters

### Background

C# union types are new in .NET 11, and a Blazor component parameter can be typed as a union. For example, `SlotContent` accepts either plain text or a Razor template:

```csharp
using Microsoft.AspNetCore.Components;

public union SlotContent(string, RenderFragment);
```

A templated component can expose a parameter of that union type:

```razor
@code {
[Parameter, EditorRequired]
public SlotContent Content { get; set; }
}
```

The `RenderFragment` case can currently be assigned directly:

```razor

```

Razor normally allows a component parameter of type `RenderFragment` or `RenderFragment` to be supplied using a child element whose name matches the template parameter:

```razor

@currentCount

```

This is the same templated-component syntax used for parameters such as `ItemTemplate`, `Authorized`, and `LoggingIn`.

### Problem

The named `` template doesn't work when the declared parameter type is a union containing a `RenderFragment` case.

Razor only recognizes the element as a template parameter when the parameter itself is typed as `RenderFragment` or `RenderFragment`. It doesn't recognize a union containing one of those types, so the generated fragment isn't assigned through the union's implicit conversion.

This issue concerns named template parameters. It isn't about the conventional `ChildContent` parameter used for direct, unwrapped component content.

### Request

Support named template elements for union-typed component parameters that include a compatible `RenderFragment` or `RenderFragment` case.

For example, Razor should compile:

```razor

@currentCount

```

by generating a `RenderFragment` for the `` element and assigning it through the union's implicit conversion.

If this isn't feasible, provide a clear compiler diagnostic that explains that the union's fragment case must be assigned directly.

### Related work

- String-literal attributes for unions containing a `string` case: dotnet/razor#13188
- ASP.NET Core union support: dotnet/aspnetcore#64599
- Blazor union verification: dotnet/aspnetcore#67296
- Union specification: aspnet/specs#782

Start with Razor's templated-component parameter recognition and compare the related union-support issues dotnet/razor#13188, dotnet/aspnetcore#64599, dotnet/aspnetcore#67296, and aspnet/specs#782. Done means the named example compiles by converting its generated RenderFragment through the union, or emits a clear diagnostic if unsupported.

Written by the indexing model from the issue text.