Fix: Move -Wwarning flag to conditional compilation to avoid Xcode conflict

[Kyle-Ye]

Summary

Moves the -Wwarning DeprecatedDeclaration compiler flag from default settings to conditional compilation (only enabled when OPENSWIFTUI_WERROR=1).

Problem

When OpenSwiftUI is used as a non-local dependency in Xcode, the -Wwarning flag conflicts with Xcode's -suppress-warnings flag, causing a build error:

Conflicting options '-Wwarning' and '-suppress-warnings'

Solution

• Removed -Wwarning DeprecatedDeclaration from default sharedSwiftSettings
• Added it to the warningsAsErrorsCondition block instead
• This preserves the warning behavior in CI/strict builds while avoiding conflicts in dependent projects

References

• Swift Forums discussion: https://forums.swift.org/t/warnings-as-errors-in-sub-packages/70810

Test Plan

• Builds successfully when used as a local dependency
• Builds successfully when used as a non-local dependency in Xcode
• Deprecated API warnings still show when OPENSWIFTUI_WERROR=1 is set

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 24.65%. Comparing base (39691cb) to head (a601d7d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #552      +/-   ##
==========================================
+ Coverage   16.43%   24.65%   +8.22%     
==========================================
  Files         481      505      +24     
  Lines       26838    29204    +2366     
==========================================
+ Hits         4410     7200    +2790     
+ Misses      22428    22004     -424     

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