Enable nullable reference types in AidlCompiler.cs

[Copilot]

AidlCompiler.cs had several fields, parameters, and an event typed as non-nullable despite accepting null values, hiding potential null reference issues at compile time.

• Add #nullable enable directive
• Annotate output_path, output_ns as string?
• Make FileWritten event nullable; use null-conditional invoke to prevent NullReferenceException when no subscribers are attached:

  FileWritten?.Invoke (file, csharp);

• Fix Run test overload signature: out string? output, AssemblyDefinition[]? references
• No ! (null-forgiving) operator used