Add SonarAnalyzer.CSharp and fix S3267 loop simplification

[Copilot]

Adds SonarAnalyzer.CSharp v10.16.1.129956 as an analyzer dependency to both library and test projects. Addresses one violation (S3267) requiring loop simplification via LINQ Where.

Changes

• Package references: Added SonarAnalyzer.CSharp to both csproj files with PrivateAssets to keep it as a build-time analyzer
• SpdxCreationInformation.cs: Refactored creator validation loop to use Where filter

Before

foreach (var creator in Creators)
    if (!creator.StartsWith("Person:") &&
        !creator.StartsWith("Organization:") &&
        !creator.StartsWith("Tool:"))
        issues.Add($"Document Invalid Creator Entry '{creator}'");

After

foreach (var creator in Creators.Where(c =>
    !c.StartsWith("Person:") &&
    !c.StartsWith("Organization:") &&
    !c.StartsWith("Tool:")))
{
    issues.Add($"Document Invalid Creator Entry '{creator}'");
}

Original prompt

please add the SonarAnalyzer.CSharp dependency to the csproj files and fix any issues it causes.

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