feat: add message field to ProgressNotification according to 2025-03-26 spec

src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/Protocol.kt

@@ -260,6 +260,7 @@ public abstract class Protocol(
         LOGGER.trace { "Received progress notification: token=${notification.progressToken}, progress=${notification.progress}/${notification.total}" }
         val progress = notification.progress
         val total = notification.total
+        val message = notification.message
         val progressToken = notification.progressToken
 
         val handler = progressHandlers[progressToken]
@@ -272,7 +273,7 @@ public abstract class Protocol(
             return
         }
 
-        handler.invoke(Progress(progress, total))
+        handler.invoke(Progress(progress, total, message))
     }
 
     private fun onResponse(response: JSONRPCResponse?, error: JSONRPCError?) {

src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types.kt

@@ -510,6 +510,11 @@ public sealed interface ProgressBase {
      * Total number of items to a process (or total progress required), if known.
      */
     public val total: Double?
+
+    /**
+     * An optional message describing the current progress.
+     */
+    public val message: String?
 }
 
 /* Progress notifications */
@@ -530,6 +535,11 @@ public open class Progress(
      * Total number of items to a process (or total progress required), if known.
      */
     override val total: Double?,
+
+    /**
+     * An optional message describing the current progress.
+     */
+    override val message: String?,
 ) : ProgressBase
 
 /**
@@ -546,6 +556,7 @@ public data class ProgressNotification(
     public val progressToken: ProgressToken,
     @Suppress("PropertyName") val _meta: JsonObject = EmptyJsonObject,
     override val total: Double?,
+    override val message: String?,
 ) : ClientNotification, ServerNotification, ProgressBase {
     override val method: Method = Method.Defined.NotificationsProgress
 }