Add support for message field in progress notifications

Implements the new optional message field for progress notifications as
defined in the MCP specification update. This allows servers to provide
descriptive status updates alongside progress values.

Changes:
- Add optional message field to ProgressSchema in types.ts
- Add tests for progress notifications with message field
- Update protocol tests to verify message field handling

Refs: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/197

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/server/mcp.test.ts

@@ -79,6 +79,99 @@ describe("McpServer", () => {
       }
     ])
   });
+
+  /***
+   * Test: Progress Notification with Message Field
+   */
+  test("should send progress notifications with message field", async () => {
+    const mcpServer = new McpServer(
+      {
+        name: "test server",
+        version: "1.0",
+      }
+    );
+
+    // Create a tool that sends progress updates
+    mcpServer.tool(
+      "long-operation",
+      "A long running operation with progress updates",
+      {
+        steps: z.number().min(1).describe("Number of steps to perform"),
+      },
+      async ({ steps }, { sendNotification, _meta }) => {
+        const progressToken = _meta?.progressToken;
+
+        if (progressToken) {
+          // Send progress notification for each step
+          for (let i = 1; i <= steps; i++) {
+            await sendNotification({
+              method: "notifications/progress",
+              params: {
+                progressToken,
+                progress: i,
+                total: steps,
+                message: `Completed step ${i} of ${steps}`,
+              },
+            });
+          }
+        }
+
+        return { content: [{ type: "text" as const, text: `Operation completed with ${steps} steps` }] };
+      }
+    );
+
+    const progressUpdates: Array<{ progress: number, total?: number, message?: string }> = [];
+
+    const client = new Client({
+      name: "test client",
+      version: "1.0",
+    });
+
+    const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+    await Promise.all([
+      client.connect(clientTransport),
+      mcpServer.server.connect(serverTransport),
+    ]);
+
+    // Call the tool with progress tracking
+    await client.request(
+      {
+        method: "tools/call",
+        params: {
+          name: "long-operation",
+          arguments: { steps: 3 },
+          _meta: {
+            progressToken: "progress-test-1"
+          }
+        }
+      },
+      CallToolResultSchema,
+      {
+        onprogress: (progress) => {
+          progressUpdates.push(progress);
+        }
+      }
+    );
+
+    // Verify progress notifications were received with message field
+    expect(progressUpdates).toHaveLength(3);
+    expect(progressUpdates[0]).toMatchObject({
+      progress: 1,
+      total: 3,
+      message: "Completed step 1 of 3",
+    });
+    expect(progressUpdates[1]).toMatchObject({
+      progress: 2,
+      total: 3,
+      message: "Completed step 2 of 3",
+    });
+    expect(progressUpdates[2]).toMatchObject({
+      progress: 3,
+      total: 3,
+      message: "Completed step 3 of 3",
+    });
+  });
 });
 
 describe("ResourceTemplate", () => {