Memory leaking in MarkdownTextBlock

### Describe the bug

A memory leak occurs in `MarkdownTextBlock` when the Text property is bound to a live stream.
The same update logic works correctly with `TextBox` and does not exhibit any memory issues.
- Using MarkdownTextBlock causes a memory leak.
The private bytes start at around 100 MB and grow to over 3 GB after loading the entire text.
- Using a TextBox instead of MarkdownTextBlock does not cause a memory leak.
The private bytes start at around 100 MB and only increase to about 150 MB after loading the entire text.

##### xaml code:

```xaml

```

##### csharp code:
```csharp
public partial class BaseMessage : ReactiveObject
{
[Reactive]
private string _messageText = string.Empty;
}
public partial class ModelMessage : BaseMessage
{
public Task SubscribeAsync(IAsyncEnumerable strings, CancellationToken cancellationToken)
{
var observable = strings
.ToObservable()
.Buffer(TimeSpan.FromMilliseconds(100))
.Where(buffer => buffer.Count > 0)
.ObserveOn(RxApp.MainThreadScheduler);
observable.Subscribe(buffer =>
{
MessageText += string.Concat(buffer);
});
return Task.CompletedTask;
}
}
```

```charp
public partial class ChatViewModel:ReactiveObject
{
private const string _markdown = "**"
public ObservableCollection Messages { get; } = [];

[ReactiveCommand]
public async Task SendMessage(string message)
{
var modelMessage = new ModelMessage();
Messages.Add(modelMessage);
var textStream = GetTextStream(_markdown, delayMs: 5);
_= modelMessage.SubscribeAsync(textStream,cancellationToken:System.Threading.CancellationToken.None);
}

public static async IAsyncEnumerable GetTextStream(
string text,
int delayMs = 10,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
{
foreach (var ch in text)
{
cancellationToken.ThrowIfCancellationRequested();
yield return ch.ToString();
await Task.Delay(delayMs, cancellationToken);
}
}
````

** _markdown is a sample markdown code :
https://github.com/CommunityToolkit/Labs-Windows/blob/18946f6c6d5194e0dbc90e3c541cfc5284d07c09/components/MarkdownTextBlock/samples/MarkdownTextBlockExampleSample.xaml.cs#L25

### Steps to reproduce

```text
- Create a new winui3 project
- Paste the code
- Run/Debug
- Start a memory profile tool:process informer
```

### Expected behavior

Although the memory may temporarily spike to a higher value, it eventually returns to a normal level.

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [x] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [x] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [x] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

2022

### Visual Studio Build Number

17.14.18

### Device form factor

Desktop

### Additional context

_No response_

### Help us help you

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