Fix UIHostingView environment resolution

[Kyle-Ye]

Summary

• Return the resolved and platform-configured environment from UIHostingViewBase.startUpdateEnvironment()
• Add a regression test covering trait collection resolution when an inherited environment is present

[augmentcode]

🤖 Augment PR Summary

Summary: Fixes UIHostingViewBase.startUpdateEnvironment() to return the trait-resolved, platform-configured EnvironmentValues instead of the pre-resolution base.
Changes: Adds a regression test asserting trait-collection-derived values (layout direction, display scale) override an inherited environment during environment updates.

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[augmentcode]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.93%. Comparing base (64924bc) to head (46e9874).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #920      +/-   ##
==========================================
- Coverage   25.94%   25.93%   -0.01%     
==========================================
  Files         720      720              
  Lines       50774    50774              
==========================================
- Hits        13172    13169       -3     
- Misses      37602    37605       +3     

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