feat(focus): implement focus event proxy

[Kyle-Ye]

Summary

• Add focus event proxy wrapping when focus inputs and responder preferences require it, with a stable proxy as child responders change.
• Keep the proxy responder private while preserving lookup on FocusableViewResponder through an internal cast helper.

Validation

Scoped type checks and standalone proxy behavior checks passed. A full package build was not run.

[augmentcode]

^{PR Risk Analyzer Agent 🛡️}

👀 Human review needed

Justification

This PR introduces net-new focus event proxy behavior rather than a trivial or mechanical change:

• Adds FocusEventProxyModifier (iOS/visionOS) that rewires a view's viewResponders preference via a new FocusEventProxyResponderFilter attribute-graph StatefulRule when IOSFocusEnabledFlag is set and preferences.requiresViewResponders is true.
• Makes FocusEventProxyResponder private and adds an asFocusEventProxyResponder cast helper, which FocusableViewResponder.focusProxyResponder now uses to keep the lookup working across the visibility change.

The new logic wires directly into the attribute graph that routes focus events, so a defect here could affect focus event delivery and responder resolution. The PR description also states a full package build was not run, leaving compile/runtime integration unverified. These factors place the change beyond automatic low-risk approval and warrant human review.

_{Reviewed commit ce2b835}

[augmentcode]

^{Deep Code Review Agent🐛}

Review completed with 1 suggestion.

_{Reviewed commit: ce2b835}

[tuist]

🛠️ Tuist Run Report 🛠️

Tests 🧪

Scheme	Status	Ran test modules	Commit
OpenSwiftUI	✅	7	3348e750d

Builds 🔨

Scheme	Status	Duration	Commit
OpenSwiftUI	✅	28.0s	3348e750d

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.54%. Comparing base (4fa6573) to head (ce2b835).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...rces/OpenSwiftUI/Event/Focus/FocusEventProxy.swift	0.00%	2 Missing ⚠️
Sources/OpenSwiftUI/Event/Focus/Focusable.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1132      +/-   ##
==========================================
- Coverage   30.89%   26.54%   -4.36%     
==========================================
  Files         880      821      -59     
  Lines       71040    61716    -9324     
==========================================
- Hits        21945    16380    -5565     
+ Misses      49095    45336    -3759     

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