Fix compliance findings in Frends.JSON.Validate (net8, analyzers, error handling)

[Copilot]

Resolves 5 Frends Task analyzer findings and upgrades the task to .NET 8, adding standard error-handling options in the process.

Changes

Compliance fixes

• JSON class made static (FT0004)
• CancellationToken parameter added to Validate() (FT0007)
• Error property added to Result (FT0011)
• ThrowErrorOnFailure and ErrorMessageOnFailure added to Options (FT0015 ×2)

Infrastructure

• Both projects retargeted from net6.0 → net8.0
• Version bumped 1.0.0 → 1.1.0

Error handling
Added Helpers/ErrorHandler.cs (standard pattern). Validate() now wraps all logic in a top-level try/catch:

catch (Exception ex)
{
    return ex.Handle(options); // respects ThrowErrorOnFailure / ErrorMessageOnFailure
}

The existing ThrowOnInvalidJson behaviour is preserved inside the inner try/catch.

Tests

• Existing tests updated with CancellationToken.None
• ErrorHandlerTests.cs added covering throw-on-failure, return-failed-result, and custom error message scenarios