Implement MCP task support (SEP-1686) (#1170)

Co-authored-by: Stephen Halter <halter73@gmail.com>

src/ModelContextProtocol.Core/Protocol/McpTaskStatus.cs

@@ -0,0 +1,79 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents the status of an MCP task.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Tasks progress through a defined lifecycle:
+/// <list type="bullet">
+/// <item><description><see cref="Working"/>: The request is currently being processed.</description></item>
+/// <item><description><see cref="InputRequired"/>: The receiver needs input from the requestor. 
+/// The requestor should call tasks/result to receive input requests.</description></item>
+/// <item><description><see cref="Completed"/>: The request completed successfully and results are available.</description></item>
+/// <item><description><see cref="Failed"/>: The request did not complete successfully.</description></item>
+/// <item><description><see cref="Cancelled"/>: The request was cancelled before completion.</description></item>
+/// </list>
+/// </para>
+/// <para>
+/// Terminal states are <see cref="Completed"/>, <see cref="Failed"/>, and <see cref="Cancelled"/>.
+/// Once a task reaches a terminal state, it cannot transition to any other status.
+/// </para>
+/// </remarks>
+[JsonConverter(typeof(JsonStringEnumConverter<McpTaskStatus>))]
+[Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+public enum McpTaskStatus
+{
+    /// <summary>
+    /// The request is currently being processed.
+    /// </summary>
+    /// <remarks>
+    /// Tasks begin in this status when created. From <see cref="Working"/>, tasks may transition
+    /// to <see cref="InputRequired"/>, <see cref="Completed"/>, <see cref="Failed"/>, or <see cref="Cancelled"/>.
+    /// </remarks>
+    [JsonStringEnumMemberName("working")]
+    Working,
+
+    /// <summary>
+    /// The receiver needs input from the requestor.
+    /// </summary>
+    /// <remarks>
+    /// The requestor should call tasks/result to receive input requests, even though the task
+    /// has not reached a terminal state. From <see cref="InputRequired"/>, tasks may transition
+    /// to <see cref="Working"/>, <see cref="Completed"/>, <see cref="Failed"/>, or <see cref="Cancelled"/>.
+    /// </remarks>
+    [JsonStringEnumMemberName("input_required")]
+    InputRequired,
+
+    /// <summary>
+    /// The request completed successfully and results are available.
+    /// </summary>
+    /// <remarks>
+    /// This is a terminal status. Tasks in this status cannot transition to any other status.
+    /// </remarks>
+    [JsonStringEnumMemberName("completed")]
+    Completed,
+
+    /// <summary>
+    /// The associated request did not complete successfully.
+    /// </summary>
+    /// <remarks>
+    /// This is a terminal status. For tool calls specifically, this includes cases where
+    /// the tool call result has isError set to true. Tasks in this status cannot transition
+    /// to any other status.
+    /// </remarks>
+    [JsonStringEnumMemberName("failed")]
+    Failed,
+
+    /// <summary>
+    /// The request was cancelled before completion.
+    /// </summary>
+    /// <remarks>
+    /// This is a terminal status. Tasks in this status cannot transition to any other status.
+    /// </remarks>
+    [JsonStringEnumMemberName("cancelled")]
+    Cancelled
+}