Update toolchains for Xcode 26.3 and Swift 6.2.4

[Kyle-Ye]

Summary

Close #869

• Update CI/toolchain configuration for Xcode 26.3 and Swift 6.2.4 across Darwin and Ubuntu jobs.
• Refresh OpenAttributeGraph, OpenRenderBox, and DarwinPrivateFrameworks package pins.
• Remove the obsolete Swift toolchain patch script and update compatibility tests for the Xcode 26.3 SDK/runtime behavior.

Dependency

• Update CI toolchain versions DarwinPrivateFrameworks#66
• Update CI toolchain versions OpenAttributeGraph#227
• Update CI toolchain versions OpenRenderBox#27

[augmentcode]

🤖 Augment PR Summary

Summary: Updates OpenSwiftUI’s supported toolchains and CI configuration for Xcode 26.3 and Swift 6.2.4.

Changes:

• Bump default Xcode version used by custom GitHub actions and macOS/iOS/UITest/compat workflows to 26.3.
• Update Ubuntu CI to Swift 6.2.4 and remove the now-obsolete toolchain patch step.
• Remove .github/scripts/fix-toolchain.sh (previously patching CoreFoundation headers).
• Update SwiftPM tools versions to 6.2 (root package and Tuist example) and refresh dependency pins in Package.resolved.
• Update dyld SPI shims/constants and tests to reflect Xcode 26.3 SDK/runtime versioning behavior.
• Update docs (README/INTEGRATION) to recommend the new toolchain versions.

Technical Notes: Some tests were adapted to newer Swift Testing behaviors (e.g., process-exit assertions) under the updated toolchains.

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[augmentcode]

Review completed. 2 suggestions posted.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 14.21%. Comparing base (a8bfc2a) to head (8986e78).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #899      +/-   ##
==========================================
- Coverage   14.21%   14.21%   -0.01%     
==========================================
  Files         654      654              
  Lines       42641    42641              
==========================================
- Hits         6063     6061       -2     
- Misses      36578    36580       +2     

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