Fix App renderer configuration default visibility

[Kyle-Ye]

Agent Summary

Make the default App.rendererConfiguration implementation visible to regular OpenSwiftUI imports so TestingHostApp can satisfy App without adding a local override.

The stdout renderer configuration requirement remains SPI-gated, while the default nil witness is available for conforming app types. This fixes the UI test host build regression after #893 when OSUI_UITests imports TestingHost.

Validation

• ./setup.sh from Example
• xcodebuild -workspace Example/Example.xcworkspace -scheme OSUI_UITests -destination 'generic/platform=iOS Simulator' -configuration OpenSwiftUIDebug build CODE_SIGNING_ALLOWED=NO

[Kyle-Ye]

/uitest macos

[augmentcode]

🤖 Augment PR Summary

Summary: Makes the default App.rendererConfiguration witness publicly visible (non-SPI) so regular OpenSwiftUI imports can satisfy App without a local override.
Why: Fixes the UI test host build regression introduced after #893 while keeping stdout renderer configuration features SPI-gated.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.69%. Comparing base (f46200a) to head (ec6af29).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #900   +/-   ##
=======================================
  Coverage   26.69%   26.69%           
=======================================
  Files         697      697           
  Lines       48910    48910           
=======================================
+ Hits        13057    13058    +1     
+ Misses      35853    35852    -1     

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[augmentcode]

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