fix: parse multi-digit .NET versions for host platform selection

[Copilot]

Fixes issue where SourceGenerationTask fails to correctly determine host platform for .NET 10+. The original code extracted only a single character from the framework description string, which works for .NET 6-9 but breaks for multi-digit versions.

Changes

• Version parsing: Extract version string using Substring(5).Split('.')[0] instead of single-character extraction
• Error handling: Added fallback to "net6" if version parsing fails
• Backward compatibility: Maintains support for existing .NET 6-9 versions

// Before: only worked for single-digit versions
var versionChar = fx[5];
hostPlatform = "net" + versionChar;

// After: handles multi-digit versions
var versionString = fx.Substring(5).Split('.')[0];
if (int.TryParse(versionString, out var version))
{
    hostPlatform = (version > 6) ? "net6" : "net" + version;
}
else
{
    hostPlatform = "net6"; // fallback
}

The fix ensures proper host path resolution for all .NET versions while respecting the existing <RollForward>LatestMajor</RollForward> configuration that targets net6 as the minimum shipped host platform.

Closes #167

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