Fix compliance findings in Frends.Edifact.ConvertToXml

[Copilot]

Resolves 5 analyzer findings (FT0007 ×2, FT0011 ×2, FT0012 ×1) and upgrades target framework from net6.0 to net8.0.

Breaking change — version bumped to 2.0.0.

Changes

• Target framework: net6.0 → net8.0 (both task and test projects)
• Method signature: Added required Options and CancellationToken parameters to ConvertToXml
• Result class: Added Success and Error properties
• Documentation: Added [Documentation(...)] attribute to task method
• Error handling: Added ErrorHandler + ValidationHandler helpers; all exceptions route through exception.Handle(options)
• Analyzers: Added StyleCop.Analyzers and FrendsTaskAnalyzers package references
• Tests: Updated existing tests for new signature; added ErrorHandlerTest class

New method signature

[Documentation("https://tasks.frends.com/tasks/frends-tasks/Frends.Edifact.ConvertToXml")]
public static Result ConvertToXml(
    [PropertyTab] Input input,
    [PropertyTab] Options options,
    CancellationToken cancellationToken)

Note

Build cannot be verified locally — depends on private NuGet packages (Frends.Edifabric.*) only available in CI.