FindChild Doesn't Search Expander Header

Describe the bug

The FindChild methods in FrameworkElementExtensions fail to search for FrameworkElements in the Header of an Expander control. A Framework control is treated as a ContentControl, but the library only recursively searches the Content of the ContentControl, but an ExpanderControl also has an additional Header that gets skipped.

Steps to reproduce

Repro steps:
1. Create an expander control with Header & Content.
2. Put a demo control in the Header to search for.
3. Write a test using VisualUITestBase that loads the control.
4. Attempt to search the Expander control for a sub control under the Header.
5. The search should return an empty result.

Expected behavior

I would expect to be able to find FrameworkElements in the Header of an Expander control using the FindChild methods in FrameworkElementExtensions.

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)
[ ] Windows 10 2004 (Build 19041)
[ ] Windows 10 20H2 (Build 19042)
[ ] Windows 10 21H1 (Build 19043)
[ ] Windows 10 21H2 (Build 19044)
[ ] Windows 10 22H2 (Build 19045)
[X] Windows 11 21H2 (Build 22000)
[ ] Other (specify)

Other Windows Build number

No response

App minimum and target SDK version

[ ] Windows 10, version 1809 (Build 17763)
[ ] Windows 10, version 1903 (Build 18362)
[ ] Windows 10, version 1909 (Build 18363)
[ ] Windows 10, version 2004 (Build 19041)
[ ] Windows 10, version 2104 (Build 20348)
[X] Windows 11, version 22H2 (Build 22000)
[ ] Other (specify)

Other SDK version

No response

Visual Studio Version

2022

Visual Studio Build Number

No response

Device form factor

No response

Additional context

No response

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CommunityToolkit / Windows