Fix compatibility test import flag parsing

[Kyle-Ye]

Summary

• Centralize SwiftUI compatibility test settings in a shared [SwiftSetting] helper.
• Pass the SwiftUI_SPI module search path as a single -I<path> compiler argument.
• Regenerate Package.resolved for the updated manifest.

Motivation

The macOS compatibility workflow fails under Xcode 26.3 while compiling OpenSwiftUICompatibilityTests:

error: unexpected input file: .../usr/lib/swift/host/plugins/testing

SwiftPM previously emitted the custom module search option as separate -I and path arguments. With the testing plugin options added by this toolchain, the plugin directory could be misclassified as an input file.

Passing the module search path as one argument preserves the same search behavior while avoiding the argument parsing failure. The shared helper also keeps both compatibility test targets configured consistently.

[augmentcode]

🤖 Augment PR Summary

Summary: Updates the compatibility-test targets’ SwiftPM compiler flags to pass the SwiftUI SPI module search path as a single -I<path> argument, avoiding Xcode 26.3/SwiftPM import-flag parsing failures.
Also: Centralizes the shared compatibility-test [SwiftSetting] configuration in one helper and regenerates Package.resolved accordingly.

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[augmentcode]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.90%. Comparing base (e2c69fa) to head (fe170a0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #936      +/-   ##
==========================================
- Coverage   25.92%   25.90%   -0.02%     
==========================================
  Files         721      721              
  Lines       50788    50788              
==========================================
- Hits        13165    13159       -6     
- Misses      37623    37629       +6     

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