AsAITool(BetaToolUnion) returns the union type instead of the tool name

## Bug Description

`AsAITool(BetaToolUnion)` exposes the CLR union variant name through `AITool.Name`.

For custom `BetaTool` values, every wrapped tool reports `"BetaTool"` instead of its actual model-visible name. This can cause generic Microsoft.Extensions.AI middleware to treat distinct tools as duplicates.

## Reproduction

```csharp
using Anthropic.Models.Beta.Messages;
using Microsoft.Extensions.AI;

AITool first = new BetaToolUnion(new BetaTool
{
Name = "first_tool",
InputSchema = new InputSchema(),
}).AsAITool();

AITool second = new BetaToolUnion(new BetaTool
{
Name = "second_tool",
InputSchema = new InputSchema(),
}).AsAITool();

Console.WriteLine(first.Name); // BetaTool
Console.WriteLine(second.Name); // BetaTool
```

## Expected Behavior

```text
first_tool
second_tool
```

## Root Cause

The wrapper currently returns the variant's CLR type:

```csharp
public override string Name =>
tool.Value?.GetType().Name ?? base.Name;
```

For custom tools, it should return `BetaTool.Name`.

The underlying union is serialized correctly; this issue only affects the `AITool` metadata exposed to middleware.

## Environment

- Anthropic 12.42.0
- .NET 10