Typed identifiers generated in global namespace fail to compile

When typed id is placed in the global namespace, the project with generated code fails to compile with rather unhelpful build report.

using LeanCode.DomainModels.Ids;

// note no namespace declaration

[TypedId(TypedIdFormat.RawGuid)]
public readonly partial record struct Id;

results with

Restore complete (0.5s)
  Example failed with 7 error(s) (0.4s)
    /home/doktornauk/example/Example/src/obj/Debug/net9.0/LeanCode.DomainModels.Generators/LeanCode.DomainModels.Generators.TypedIdGenerator/Id.g.cs(3,11): error CS1001: Identifier expected
    /home/doktornauk/example/Example/src/obj/Debug/net9.0/LeanCode.DomainModels.Generators/LeanCode.DomainModels.Generators.TypedIdGenerator/Id.g.cs(3,11): error CS1514: { expected
    /home/doktornauk/example/Example/src/obj/Debug/net9.0/LeanCode.DomainModels.Generators/LeanCode.DomainModels.Generators.TypedIdGenerator/Id.g.cs(3,12): error CS0116: A namespace cannot directly contain members such as fields, methods or statements
    /home/doktornauk/example/Example/src/obj/Debug/net9.0/LeanCode.DomainModels.Generators/LeanCode.DomainModels.Generators.TypedIdGenerator/Id.g.cs(3,28): error CS1001: Identifier expected
    /home/doktornauk/example/Example/src/obj/Debug/net9.0/LeanCode.DomainModels.Generators/LeanCode.DomainModels.Generators.TypedIdGenerator/Id.g.cs(3,28): error CS1514: { expected
    /home/doktornauk/example/Example/src/obj/Debug/net9.0/LeanCode.DomainModels.Generators/LeanCode.DomainModels.Generators.TypedIdGenerator/Id.g.cs(4,1): error CS1022: Type or namespace definition, or end-of-file expected
    /home/doktornauk/example/Example/src/obj/Debug/net9.0/LeanCode.DomainModels.Generators/LeanCode.DomainModels.Generators.TypedIdGenerator/Id.g.cs(77,2): error CS1513: } expected

Build failed with 7 error(s) in 1.1s

This result comes from using ISymbol.ContainingNamespace.ToDisplayString() here, which resolves to <global namespace> for the global namespace, which is not a valid identifier in C# code.

We should either properly support identifiers in the global namespace or emit a diagnostic report instead of non-compiling code (preferably maybe?).

Start in src/Domain/LeanCode.DomainModels.Generators/TypedIdGenerator.cs around the use of ISymbol.ContainingNamespace.ToDisplayString(). Reproduce the global-namespace TypedId example and build it to confirm the generated Id.g.cs failure. Done means global-namespace identifiers are either supported without compilation errors or a clear diagnostic is emitted instead of non-compiling generated code.