Clean up namespaces (#410)

No meaningful code changes, just lots of namespace shuffling to have fewer namespaces and to put things in better namespaces.

src/ModelContextProtocol/Protocol/EmptyResult.cs

@@ -0,0 +1,13 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents an empty result object for operations that need to indicate successful completion 
+/// but don't need to return any specific data.
+/// </summary>
+public class EmptyResult
+{
+    [JsonIgnore]
+    internal static EmptyResult Instance { get; } = new();
+}