VisualDiagnostics.RegisterSourceInfo returns null if debugger is not attached

### Description

XAML Hot Reload not available when attaching to MAUI+WinUI process even if having system level env var ENABLE_XAML_DIAGNOSTICS_SOURCE_INFO=1.

### Steps to Reproduce

1. Set system wide env var ENABLE_XAML_DIAGNOSTICS_SOURCE_INFO=1
2. Open VS and create MAUI app.
3. Build.
4. Run without debugger (i.e. Ctrl+F5).
5. Debugger > Attach to Process, select MAUI app started on prev step.
6. Open Live Visual Tree, make sure Just My XAML is turned ON.

**Actual**: LVT is empty
**Expected**: LVT is not empty
**Note**: MAUI needs to update all places where it checks for DebuggerHelper.IsDebuggerAttached and switch to checking ENABLE_XAML_DIAGNOSTICS_SOURCE_INFO.

### Link to public reproduction project repository

n/a

### Version with bug

6.0 Release Candidate 3

### Last version that worked well

Unknown/Other

### Affected platforms

Windows, I was *not* able test on other platforms

### Affected platform versions

all

### Did you find any workaround?

_No response_

### Relevant log output

_No response_