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/shared/protocol.test.ts

@@ -255,6 +255,74 @@ describe("protocol tests", () => {
       await Promise.resolve();
       await expect(requestPromise).resolves.toEqual({ result: "success" });
     });
+
+    test("should handle progress notifications with message field", async () => {
+      await protocol.connect(transport);
+      const request = { method: "example", params: {} };
+      const mockSchema: ZodType<{ result: string }> = z.object({
+        result: z.string(),
+      });
+      const onProgressMock = jest.fn();
+
+      const requestPromise = protocol.request(request, mockSchema, {
+        timeout: 1000,
+        onprogress: onProgressMock,
+      });
+
+      jest.advanceTimersByTime(200);
+
+      if (transport.onmessage) {
+        transport.onmessage({
+          jsonrpc: "2.0",
+          method: "notifications/progress",
+          params: {
+            progressToken: 0,
+            progress: 25,
+            total: 100,
+            message: "Initializing process...",
+          },
+        });
+      }
+      await Promise.resolve();
+
+      expect(onProgressMock).toHaveBeenCalledWith({
+        progress: 25,
+        total: 100,
+        message: "Initializing process...",
+      });
+
+      jest.advanceTimersByTime(200);
+
+      if (transport.onmessage) {
+        transport.onmessage({
+          jsonrpc: "2.0",
+          method: "notifications/progress",
+          params: {
+            progressToken: 0,
+            progress: 75,
+            total: 100,
+            message: "Processing data...",
+          },
+        });
+      }
+      await Promise.resolve();
+
+      expect(onProgressMock).toHaveBeenCalledWith({
+        progress: 75,
+        total: 100,
+        message: "Processing data...",
+      });
+
+      if (transport.onmessage) {
+        transport.onmessage({
+          jsonrpc: "2.0",
+          id: 0,
+          result: { result: "success" },
+        });
+      }
+      await Promise.resolve();
+      await expect(requestPromise).resolves.toEqual({ result: "success" });
+    });
   });
 });