ContentView's OnSizeAllocated method will loaded before controls init in xaml

### Description

Today i create a ContentView, i want get control size at OnSizeAllocated, but when app first time load OnSizeAllocated, control is null. I try to do this at ContentPage, OnSizeAllocated will get correct control, so i feel it is a bug.

### Steps to Reproduce

1. Create a Maui app project.
2. Create a ContentView and add it to MainPage, and create OnSizeAllocated method, create control in xaml, and use control in OnSizeAllocated method.
3. set break point in OnSizeAllocated method,then run

### Version with bug

6.0 (current)

### Last version that worked well

Release Candidate 3 (current)

### Affected platforms

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

### Affected platform versions

Windows SDK 10.0.22000.194

### Did you find any workaround?

_No response_

### Relevant log output

_No response_

Reproduce the issue in a .NET MAUI app on Windows using a ContentView added to MainPage, with a XAML control accessed from OnSizeAllocated. Compare the ContentView callback with ContentPage behavior and trace the control-initialization lifecycle; done means the callback no longer exposes an uninitialized control, or the expected lifecycle behavior is clearly established.

Written by the indexing model from the issue text.