Add context to complete requests (#488)

src/ModelContextProtocol.Core/Protocol/CompleteContext.cs

@@ -0,0 +1,19 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents additional context information for completion requests.
+/// </summary>
+/// <remarks>
+/// This context provides information that helps the server generate more relevant 
+/// completion suggestions, such as previously resolved variables in a template.
+/// </remarks>
+public class CompleteContext
+{
+    /// <summary>
+    /// Gets or sets previously-resolved variables in a URI template or prompt.
+    /// </summary>
+    [JsonPropertyName("arguments")]
+    public IDictionary<string, string>? Arguments { get; init; }
+}