Update UIHostingView

[Kyle-Ye]

No description provided.

[augmentcode]

🤖 Augment PR Summary

Summary: This PR expands UIKit hosting support by refactoring and extending _UIHostingView.

Changes:

• Moves several protocol conformances for _UIHostingView into a new UIHostingView+Extension.swift file (e.g. ViewRendererHost, TestHost, HostingViewProtocol).
• Adds a ViewGraphRenderObserver.didRender() hook that forwards to UIHostingController.didRender().
• Introduces background-color management changes, including an overridden backgroundColor setter and container-background fallback properties.
• Adds UIHostingViewDelegate (package) for hosting view lifecycle/environment/preference callbacks.
• Moves/renames UIViewControllerProvider into the UIKit Controller area and narrows it to package visibility.
• Implements UIHostingViewBase.updateSize() to set the proposed size from the underlying UIKit view.
• Adjusts visibility for some integration types (e.g. PlatformItemList, RootTransformProvider) to package.

Technical Notes: Several sections remain marked WIP/unimplemented; the refactor appears to prepare for a more complete UIKit update/render cycle implementation.

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[augmentcode]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 14.28%. Comparing base (f04794e) to head (18237c0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #703      +/-   ##
==========================================
- Coverage   14.30%   14.28%   -0.02%     
==========================================
  Files         563      563              
  Lines       33190    33190              
==========================================
- Hits         4747     4742       -5     
- Misses      28443    28448       +5     

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