Generate valid C# for .proto files with no package or csharp_namespace

[pepone]

Summary

• When a .proto has neither package nor option csharp_namespace = ..., GetCsharpNamespace returned "" and the generator emitted namespace ; (CS1001 from the C# compiler) and global::.MyMessage for any cross-namespace reference.
• Skip the namespace declaration when the namespace is empty, and emit global::MyMessage instead of global::.MyMessage for cross-namespace references targeting a file with no namespace. Matches Google's protoc-gen-csharp behavior of placing top-level types in the global namespace.
• Add a .proto regression file with neither package nor csharp_namespace. Without the fix the test project fails to compile with CS1001: Identifier expected on the generated namespace ; line; with the fix it builds cleanly and all 48 Protobuf tests pass.

Fixes #4447

[Copilot]

Pull request overview

This PR fixes protoc-gen-icerpc-csharp output for valid .proto files that omit both package and option csharp_namespace, ensuring generated C# compiles and matches protoc-gen-csharp behavior for global-namespace types.

Changes:

• Skip emitting a file-scoped namespace ...; declaration when the computed C# namespace is empty.
• Fix cross-namespace message type qualification to avoid emitting invalid global::.Type when targeting a no-namespace .proto.
• Add a regression .proto with no package and no csharp_namespace to ensure the test project compiles.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/IceRpc.Protobuf.Tests/NoNamespaceTests.proto	Adds a .proto schema with no namespace metadata to catch invalid generated namespace ; output during compilation.
src/IceRpc.Protobuf.Generator/Program.cs	Conditionally emits the namespace declaration only when non-empty.
src/IceRpc.Protobuf.Generator/MessageDescriptorExtensions.cs	Centralizes message type qualification and handles the empty-namespace case to produce valid global::Type references.

[Copilot]

This regression .proto doesn’t currently exercise the cross-namespace reference scenario described in the header comment: nothing imports this file from a non-empty package/csharp_namespace, so the generator’s global::Type (vs global::.Type) path won’t be compiled/tested. Consider adding a second .proto with a non-empty package/csharp_namespace that imports this file and uses NoNamespaceMessage as an RPC input/output to ensure the generated C# contains global::NoNamespaceMessage and stays valid.