Replace ProtoFileSourceGenerator and WriteProtoFileTask with a new
GenerateProtoFileTask that creates its own Roslyn compilation. This
solves the timing issue where source generators run too late for
Grpc.Tools to process the generated proto files.
The new task runs after ResolveAssemblyReferences but before
_gRPC_GetProtoc and CoreCompile, ensuring the proto file exists
when Grpc.Tools needs it.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
