Fix SvgSource invariant globalization startup failure

[wieslawsoltes]

Summary

Fixes #541 by making SvgService safe to initialize when .NET runs in invariant globalization mode.

The previous SvgService static field eagerly called CultureInfo.GetCultureInfo("en-US"). In invariant globalization mode, named culture lookup throws CultureNotFoundException, which caused the whole SvgService type initializer to fail before an SVG could be loaded. That surfaced through Svg.Controls.Skia.Avalonia as a TypeInitializationException from SvgSource.Load(...).

This change keeps the existing deterministic en-US default for SVG systemLanguage matching, but avoids doing a throwing named-culture lookup during static initialization. If the runtime cannot create the named culture, SvgService falls back to the language tag string for conditional processing while still allowing tests and callers to override s_systemLanguageOverride.

Changes

• Added a non-throwing culture lookup helper for the default en-US system language.
• Centralized current system-language tag selection so invariant globalization still resolves the default en-US behavior.
• Added an invariant-globalization test project for Svg.Controls.Skia.Avalonia.
• Covered both inline SVG loading and file-path SVG loading through SvgSource.
• Added coverage that systemLanguage="en-US" still selects the expected branch under invariant globalization.

Validation

• dotnet format Svg.Skia.slnx --no-restore
• dotnet build Svg.Skia.slnx -c Release
• dotnet test Svg.Skia.slnx -c Release
• dotnet test tests/Svg.Controls.Skia.Avalonia.InvariantGlobalization.UnitTests/Svg.Controls.Skia.Avalonia.InvariantGlobalization.UnitTests.csproj -c Release --filter "FullyQualifiedName~SvgSourceInvariantGlobalizationTests"
• dotnet test tests/Svg.Model.UnitTests/Svg.Model.UnitTests.csproj -f net10.0 -c Release --no-restore --filter "FullyQualifiedName~SvgConditionalProcessingTests"