Add SymbolVariants API support

[Kyle-Ye]

No description provided.

[augmentcode]

🤖 Augment PR Summary

Summary: This PR introduces the SwiftUI-compatible SymbolVariants API surface in OpenSwiftUI, enabling environment-driven selection of SF Symbols variants.

Changes:

• Added SymbolVariants (Hashable/Sendable) with support for shape variants (circle/square/rectangle) and flag variants (fill, slash, and SPI background).
• Implemented variant composition semantics via formUnion/contains so repeated modifiers accumulate and the closest shape modifier wins.
• Added a View.symbolVariant(_:) modifier that updates EnvironmentValues.symbolVariants using transformEnvironment.
• Introduced environment keys for symbolVariants plus SPI v4 values (symbolBackgroundCornerRadius, symbolsGrowToFitBackground).
• Included internal helpers to derive shape suffixes and generate paths for symbol backgrounds.

Technical Notes: The implementation focuses on API + environment plumbing (v3+ public surface, v4 SPI additions) and sets defaults to .none with additive composition behavior.

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[augmentcode]

Review completed. 1 suggestions posted.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 14.43%. Comparing base (d0249df) to head (2b65fba).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...es/OpenSwiftUICore/View/Image/SymbolVariants.swift	0.00%	54 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #719      +/-   ##
==========================================
- Coverage   14.46%   14.43%   -0.04%     
==========================================
  Files         565      566       +1     
  Lines       33335    33389      +54     
==========================================
- Hits         4822     4819       -3     
- Misses      28513    28570      +57     

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