Caret Browsing causes app to crash on TextBlock with links

### Describe the bug

If Caret Browsing is enabled (by pressing F7) the app will crash whenever the user tabs on a `(Rich)TextBlock` if it contains any `Hyperlink` and `IsTextSelectionEnabled` is set to false.

### Why is this important?

Makes apps hard to use for screen reader users.
This currently accounts for more than 2.5% of crashes in my app.

### Steps to reproduce the bug

1. Add a `TextBlock` with a `Hyperlink` to a `Window`
2. Enable Caret Browsing by pressing F7
3. Tab to focus the `TextBlock`

### Actual behavior

_No response_

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

None

### Windows version

_No response_

### Additional context

This is a quite ancient bug, happens on UWP since a very long time. WinUI 3 is affected too.

Start by reproducing the crash with a (Rich)TextBlock containing a Hyperlink, IsTextSelectionEnabled set to false, Caret Browsing enabled, and focus moved with Tab. Trace the TextBlock/Hyperlink focus path and add a regression test covering this sequence; done means tabbing to the control no longer crashes and caret browsing remains usable.

Written by the indexing model from the issue text.