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/CompletionsCapability.cs

@@ -0,0 +1,37 @@
+using ModelContextProtocol.Server;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents the completions capability for providing auto-completion suggestions
+/// for prompt arguments and resource references.
+/// </summary>
+/// <remarks>
+/// <para>
+/// When enabled, this capability allows a Model Context Protocol server to provide 
+/// auto-completion suggestions. This capability is advertised to clients during the initialize handshake.
+/// </para>
+/// <para>
+/// The primary function of this capability is to improve the user experience by offering
+/// contextual suggestions for argument values or resource identifiers based on partial input.
+/// </para>
+/// <para>
+/// See the <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">schema</see> for details.
+/// </para>
+/// </remarks>
+public class CompletionsCapability
+{
+    // Currently empty in the spec, but may be extended in the future.
+
+    /// <summary>
+    /// Gets or sets the handler for completion requests.
+    /// </summary>
+    /// <remarks>
+    /// This handler provides auto-completion suggestions for prompt arguments or resource references in the Model Context Protocol.
+    /// The handler receives a reference type (e.g., "ref/prompt" or "ref/resource") and the current argument value,
+    /// and should return appropriate completion suggestions.
+    /// </remarks>
+    [JsonIgnore]
+    public Func<RequestContext<CompleteRequestParams>, CancellationToken, ValueTask<CompleteResult>>? CompleteHandler { get; set; }
+}