Fix InvalidCastException in ComboBoxAutomationPeer / forward Scroll to ItemsControl

[h3xds1nz]

Description

Fixes InvalidCastException in ComboBoxAutomationPeer which occurs because ComboBoxAutomationPeer nor its parents implement IScrollProvider directly and hence the whole code block is just wrong as it will never work.

We should use the ItemsControlAutomationPeer implementation to provide the interface via ScrollHost / ScrollViewer, which should be done only if the drop-down is open. See https://learn.microsoft.com/en-us/windows/win32/winauto/uiauto-supportcomboboxcontroltype.

Reproduction steps are easy:

1. Put a ComboBox onto your window with some items
2. Open the app, optionally enable native managed boundary exceptions in VS
3. Start Narrator or any other UIA app, and watch the exceptions flow

Customer Impact

Better performance and correct interface when the dropdown is open, improved accessibility.

Regression

Yes, this was introduced in #1278, guess the comment was misunderstood by the implementor and nobody bothered to check/test during review.

Testing

Local build, exploring the automation tree, ensuring the exception no longer happens.

Risk

Low.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 2.98819%. Comparing base (57fe8ac) to head (46bc68e).
⚠️ Report is 194 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main     #10877          +/-   ##
===================================================
- Coverage   13.48885%   2.98819%   -10.50066%     
===================================================
  Files           3317       3319           +2     
  Lines         664749     664885         +136     
  Branches       74643      74673          +30     
===================================================
- Hits           89667      19868       -69799     
- Misses        572488     644836       +72348     
+ Partials        2594        181        -2413     

Flag	Coverage Δ
Debug	2.98819% <0.00000%> (-10.50066%)	⬇️

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