Typed identifiers generated in global namespace fail to compile

[mishioo]

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?).