Fix SVG C# codegen gradient output

[wieslawsoltes]

Summary

Fixes #460.

This PR fixes two SVG C# code generation failures reported in issue #460:

• generated font family arrays now escape embedded double quotes correctly for C# verbatim string literals
• generated gradient code no longer emits invalid float literals such as NaNf

It also fixes the renderer path that produced non-finite gradient stop positions for zero-width geometry bounds, so retained shader models are built with valid, clamped SVG stop offsets before code generation runs.

Root Cause

The C# generator emitted verbatim strings with raw embedded quotes. For a font family such as "Black Ops One", this produced invalid generated C# like @""Black Ops One"" instead of doubling quotes inside the verbatim string.

Gradient color positions had two related problems:

• ToFloatArray appended f to every formatted float, which made non-finite values compile as invalid C# tokens such as NaNf.
• Gradient stop offsets were computed by converting the stop offset through ToDeviceValue(...) and dividing by skBounds.Width. For zero-width bounds, percentage stops could become 0 / 0, producing NaN positions.

Changes

• Emit valid C# constants for non-finite floats:
  • float.NaN
  • float.PositiveInfinity
  • float.NegativeInfinity
• Escape embedded quotes in generated verbatim string arrays.
• Generate null gradient color positions when retained shader positions are missing or non-finite, matching SkiaSharp's optional color-position behavior.
• Compute SVG gradient stop offsets directly from the stop offset value:
  • percentages are normalized to 0..1
  • user values are treated as already normalized
  • non-finite and out-of-range values are clamped
• Align retained shader model and runtime Skia conversion behavior so null color positions remain valid instead of dropping the shader.
• Add regression tests for string escaping, special float literals, non-finite gradient positions, zero-width gradient bounds, and runtime conversion of optional gradient positions.

Validation

• dotnet format Svg.Skia.slnx --no-restore
• dotnet test tests/Svg.Skia.UnitTests/Svg.Skia.UnitTests.csproj -f net10.0 -c Release --no-restore --filter "FullyQualifiedName~SkiaCSharpCodeGenTests" --logger "console;verbosity=normal"
• dotnet build Svg.Skia.slnx -c Release
• dotnet test Svg.Skia.slnx -c Release --no-restore

Full solution tests passed. The main Svg.Skia.UnitTests run ended with 1,640 passed, 593 skipped, and 0 failed.