Add server "completions" capability (#232)

* Split apart Capabilities.cs

No code changes, just moving each type to its own file.

* Add server completions capability

src/ModelContextProtocol/Server/McpServer.cs

@@ -175,11 +175,18 @@ internal sealed class McpServer : McpEndpoint, IMcpServer
 
     private void SetCompletionHandler(McpServerOptions options)
     {
+        if (options.Capabilities?.Completions is not { } completionsCapability)
+        {
+            return;
+        }
+
+        var completeHandler = completionsCapability.CompleteHandler ?? 
+            throw new McpException("Completions capability was enabled, but Complete handler was not specified.");
+
         // This capability is not optional, so return an empty result if there is no handler.
-        RequestHandlers.Set(RequestMethods.CompletionComplete,
-            options.GetCompletionHandler is { } handler ?
-                (request, cancellationToken) => handler(new(this, request), cancellationToken) :
-                (request, cancellationToken) => Task.FromResult(new CompleteResult() { Completion = new() { Values = [], Total = 0, HasMore = false } }),
+        RequestHandlers.Set(
+            RequestMethods.CompletionComplete,
+            (request, cancellationToken) => completeHandler(new(this, request), cancellationToken),
             McpJsonUtilities.JsonContext.Default.CompleteRequestParams,
             McpJsonUtilities.JsonContext.Default.CompleteResult);
     }

src/ModelContextProtocol/Server/McpServerHandlers.cs

@@ -43,9 +43,9 @@ public sealed class McpServerHandlers
     public Func<RequestContext<ReadResourceRequestParams>, CancellationToken, Task<ReadResourceResult>>? ReadResourceHandler { get; set; }
 
     /// <summary>
-    /// Gets or sets the handler for get completion requests.
+    /// Gets or sets the handler for completion complete requests.
     /// </summary>
-    public Func<RequestContext<CompleteRequestParams>, CancellationToken, Task<CompleteResult>>? GetCompletionHandler { get; set; }
+    public Func<RequestContext<CompleteRequestParams>, CancellationToken, Task<CompleteResult>>? CompleteHandler { get; set; }
 
     /// <summary>
     /// Gets or sets the handler for subscribe to resources messages.
@@ -109,12 +109,18 @@ public sealed class McpServerHandlers
             loggingCapability.SetLoggingLevelHandler = SetLoggingLevelHandler;
         }
 
+        CompletionsCapability? completionsCapability = options.Capabilities?.Completions;
+        if (CompleteHandler is not null)
+        {
+            completionsCapability ??= new();
+            completionsCapability.CompleteHandler = CompleteHandler;
+        }
+
         options.Capabilities ??= new();
         options.Capabilities.Prompts = promptsCapability;
         options.Capabilities.Resources = resourcesCapability;
         options.Capabilities.Tools = toolsCapability;
         options.Capabilities.Logging = loggingCapability;
-
-        options.GetCompletionHandler = GetCompletionHandler ?? options.GetCompletionHandler;
+        options.Capabilities.Completions = completionsCapability;
     }
 }

src/ModelContextProtocol/Server/McpServerOptions.cs

@@ -35,10 +35,4 @@ public class McpServerOptions
     /// Optional server instructions to send to clients
     /// </summary>
     public string ServerInstructions { get; set; } = string.Empty;
-
-    /// <summary>
-    /// Gets or sets the handler for get completion requests.
-    /// </summary>
-    [JsonIgnore]
-    public Func<RequestContext<CompleteRequestParams>, CancellationToken, Task<CompleteResult>>? GetCompletionHandler { get; set; }
 }