Add ModelContextProtocol.Core package (#428)

src/ModelContextProtocol.Core/ProgressNotificationValue.cs

@@ -0,0 +1,26 @@
+namespace ModelContextProtocol;
+
+/// <summary>Provides a progress value that can be sent using <see cref="IProgress{ProgressNotificationValue}"/>.</summary>
+public class ProgressNotificationValue
+{
+    /// <summary>
+    /// Gets or sets the progress thus far.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This value typically represents either a percentage (0-100) or the number of items processed so far (when used with the <see cref="Total"/> property).
+    /// </para>
+    /// <para>
+    /// When reporting progress, this value should increase monotonically as the operation proceeds.
+    /// Values are typically between 0 and 100 when representing percentages, or can be any positive number
+    /// when representing completed items in combination with the <see cref="Total"/> property.
+    /// </para>
+    /// </remarks>
+    public required float Progress { get; init; }
+
+    /// <summary>Gets or sets the total number of items to process (or total progress required), if known.</summary>
+    public float? Total { get; init; }
+
+    /// <summary>Gets or sets an optional message describing the current progress.</summary>
+    public string? Message { get; init; }
+}