Move XSD manifest retrieval into dedicated SlnxXsdProvider class with interface

[Copilot]

XsdValidator was directly responsible for both locating the embedded XSD resource and performing validation — mixing resource lookup with validation logic.

Changes

• New ISlnxXsdProvider — interface defining the GetXsdStream() contract
• New SlnxXsdProvider — implements ISlnxXsdProvider; extracts the const XsdResourceName and GetXsdStream() (wrapping Assembly.GetManifestResourceStream) into its own class
• XsdValidator — now accepts ISlnxXsdProvider as a constructor dependency; delegates resource loading entirely to it
• DI registration — ISlnxXsdProvider registered as singleton mapping to SlnxXsdProvider in ServiceCollectionExtensions
• Tests — updated call sites to supply new SlnxXsdProvider() when constructing XsdValidator directly

// Before — resource retrieval embedded in XsdValidator
private const string XsdResourceName = "JulianVerdurmen.SlnxValidator.Slnx.xsd";

await using var xsdStream = Assembly
    .GetExecutingAssembly()
    .GetManifestResourceStream(XsdResourceName) ?? throw new InvalidOperationException("XSD file not found");

// After — delegated via ISlnxXsdProvider
public interface ISlnxXsdProvider
{
    Stream GetXsdStream();
}

public sealed class SlnxXsdProvider : ISlnxXsdProvider
{
    private const string XsdResourceName = "JulianVerdurmen.SlnxValidator.Slnx.xsd";

    public Stream GetXsdStream() =>
        Assembly.GetExecutingAssembly()
            .GetManifestResourceStream(XsdResourceName)
            ?? throw new InvalidOperationException($"XSD resource '{XsdResourceName}' not found in assembly");
}

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[304NotModified]

@copilot create also an interface and use that one

[Copilot]

@copilot create also an interface and use that one

Done in e08b310. Added ISlnxXsdProvider interface, SlnxXsdProvider now implements it, XsdValidator depends on the interface, and DI registers ISlnxXsdProvider → SlnxXsdProvider.